MapleStory/Thief

Thieves are a semi popular class. They have their likenesses with archers and warriors. They excel at killing enemies quickly, and thus gain lots of mesos in the process. The downside is that the Thief class demands precision when allocating stats, as they can be screwed up very easily.

There are three types of Adventurer Thieves: Assassins, Bandits, and Dual Blades. Assassins are far ranged attackers. They are quite Meso efficient and have good damage potential, but cost more than bandits. Bandits are quick melee attackers and are quite powerful among the 2nd jobs. They aren't as Meso efficient as Assassins and do not have the benefit of ranged attack but make up for it in much greater raw power (after around level 90, there is no way that any assassin can catch up with Bandits' damage), although dagger scrolls have a potential to cost quite a bit.

Pros

All three thieves get good mobility skills such as Haste and Flash Jump. Dual Blades have those as well as Flying Assaulter, Tornado Spin, and Bloody Storm as attacks.
They have high avoidability. In fact, Dual Blades potentially have the best avoid out of all three thieves. The only thing keeping them from being completely invincible is the newly added 90% cap to avoiding attacks.
All of them are good at bossing at 4th job.

Cons

Average HP and MP, killed easily by monsters at high levels.
Very unstable damage until mastery in 2nd job, like most jobs.
Good stars can be pricey.

Equipment

If you are new to MapleStory or starting on a new server, you should consider skipping armor every five or so levels. Thief items are pricey, so save for future armor.

Beginner

After creating a new character and starting as a Beginner, do all the quests and level up to the required level to make your job advancement.

Beginner AP

Originally, when new characters were made, players had to randomly generate the starting AP of their character. This required knowledge of the stats used by the desired class, as well as patience, to achieve the optimal stats.

Now when you begin the game as a Beginner,your AP will automatically be placed into certain stats, and when you make the job advancement your character's AP will be set to the minimum stats to become that class. Any additional AP will be available for you to apply.

Planning your Thief

You now have two choices if you want to become a Bandit. Otherwise, you should be DEXless.

Become a STR bandit having LUK and STR (not recommended as it is weaker in higher levels. Plus, you should be able to get enough STR to wear a Dragon Khanjar solely via equips. There are also very few STR daggers in the higher levels.
Be DEXless (highly recommended).

Dagger Rogue (Pre-Bandits)

LUK bandits have more LUK than STR bandits, because STR Bandits have to have several points into STR to equip their weapons. Since they have more LUK, LUK Bandits typically do more damage than STR Bandits, due to LUK having much more substantial effect in their damage equation. However, LUK daggers generally cost a bit more than STR Daggers. The two also even out at levels 80 and 120, when they can acquire Khanjars and Dragon Khanjars, which have 7 and 15 attack clean, respectively.

The first few STR daggers have a speed of "faster" compared to most LUK dagger's speed of "fast." However this distinction is very minor as the only difference between "faster" and "fast" daggers is that without Dagger Booster, "faster" daggers perform normal swings faster. With Dagger Booster, there is no difference between a "fast" and "faster" dagger. They have the exact same speed properties for normal attacks and skills, due to the speed cap.

LUK daggers also seem to be far more common, and this greatly helps when it comes to that all important time of having to re-equip. Overall, most people say going the way of LUK is a more rewarding path in the long run.

Dagger Rogue (Pre-Dual Blade)

It is recommended to be full-LUK on a Dual Blade. They will not benefit from adding points into STR due to not being able to wear Shields, being able to only equip Kataras in their Shield Slot.

Playing a Dual Blade can be expensive due to the fact that several of their pre-4th job skills require Mastery Books, which can be pretty pricey. A good dagger can be expensive too. While you may have a lot of STR from equipment, there are very few high level STR daggers.

Claw Rogue (Pre-Assassin)

Thieves that use claws are called "Assassins" (the second job title, also known as "'sins"), and are long ranged attackers.

Equipment

It should be noted once more that Assassins can be very expensive. Tremendous amounts of Mesos are required to maintain a strong Assassin. The price of certain stars, such as Balanced Fury and the ones crafted by the Smithing profession, can be daunting. Sins do not have the MP regeneration of a mage, and as such burn MP Potions constantly. Assassins are a very common class on GMS (Global MapleStory), and prices for their equipment are inflated.

Funding

However, money concerns are not unique to Assassins. Archers depend primarily on Double Shot. Magicians equipment is sometimes as much or more than Assassins, though Sins must cover the cost of stars. Furthermore, it could be argued that Bandits cost far more to maintain than Assassins. They both spend a lot of money on potions due to lacking an HP recovery skill.

Tip: To save money as an assassin, especially in the earlier levels, recharge your stars more often instead of buying new stacks. To do this, go to the pots shop and click on the use tab on your side of the window. Next to the stars there will be an orange button with 'Rech.' on it. Simply click this button and you will have a full stack of stars again. The price to recharge is right next to the price to sell them. You can either do that, or purchase cheaper stars, since quality is not always better than quantity.

First Job Advancement

See Training locations for where to power level.

When you reach level 10, head to Kerning City. Find the fusion jazz bar above JM From tha Streetz, and head to the basement to find Dark Lord. Speak with him and become a Rogue. After doing so, speak with him again to get the "How to Survive" quest. You'll now have a direction to go, and some new equipment to help you level. Dark Lord will have given you both a Beginner Thief's Wrist Guard (claw weapon that throws stars) and a Beginner Thief's short sword (a dagger). Both items are Thief only and unable to be traded.

Now that you've become a Rogue, your skill choices are very important, so be careful. First you want to pick your offensive skill, so you probably want to test out the two weapons to get a feel for them. Claws can launch stars from a distance or up close. Although the short sword acts the same as before, your damage will be increased significantly. For both weapons, imagine them attacking twice as fast (or faster in the future) with the help of one of the skills you can invest in (Double Stab or Lucky Seven). If you are still doing the Kerning City quest chain, which you started with the "How to Survive", you should have some more thief gear to equip.

First job+ Advancement(Dual Blade)

Note, if you aren't a Dual Blade, skip this section. When you reach level 20, Lady Syl will contact you via lightbulb. Head to Lady Syl's Room to talk to her. She will tell you to head into the Marble Room. Attack the tables with the glass marbles using normal attacks. The table will break and the glass marble will drop. Pick up the marble and double click it (it's in your use inventory.) Repeat until you get a Mirror of Insight, and once you receive it, head back to Lady Syl. You will become a Blade Recruit.

Second Job Advancement

See Training locations for where to power level.

Go see Dark Lord in Kerning City at Level 30. He will say that you look much stronger and ready for 2nd job, and will bring you to a secret area, where you have to defeat Swamp Monsters and collect 30 Dark Marbles. They don't give any experience or drops except the Dark Marble. Once you are done, climb back up and exit the map through the portal. Take the 30 Dark Marbles back to Dark Lord and he will ask you to choose between 2 jobs, Bandit or Assassin. If you are a Dual Blade, you will talk to Lady Syl instead. Instead of Swamp Monsters, Lady Syl will send you to defeat Shadow Blade Lords for the Dark Marbles. Contrary to the Swamp Monsters, these do give experience. Once you finish, you will become a Blade Acolyte.

Second job+ Advancement(Dual Blades)

Note, if you aren't a Dual Blade, skip this section. The 2nd job+ advancement is simple. Once you hit level 45, Lady Syl will contact you via lightbulb again. Head to her, and she will tell you to slay 99 Ice Drakes. Head back to Lady Syl, and you will become a Blade Specialist.

Third Job Advancement

See Training locations for where to power level.

Once you are level 60, go to El Nath, and head to the house on the big hill (Chief Residence). Head to Arec, and talk with him. After a chat, head to Sharp Cliff I. Go inside the crystal portal at the bottom of the map. Double click on the Holy Stone at the bottom. Head inside, fight the Dark Lord's evil side, and head out with the Black Charm that it drops. Head back to Arec and give him this item, and voila, you are third job! Note: Dual Blades have the same advancement.

Fourth Job Advancement

See Training locations for where to power level.

Congratulations on hitting 100! Once you hit level 100, Arec the 3rd job instructor will contact you via lightbulb. After starting the quest, head to Leafre and talk to Hellin in The Forest of the Priest. You will then get a quest to obtain two special items for the fourth job instructor to advance. You will have to kill Manon and Griffey to get the two items, but thankfully, they're special versions of the bosses, which you will find instantly upon entering their respective boss map. Note, they don't drop any of the items the real versions do. After hunting down these monsters and obtaining the two items, go talk to Hellin. She will advance you to fourth job, give you the skill book for Maple Warrior 10, and give you 4 SP and 5 AP.

General Equipment and items

Level 1-10: Stick with your newbie clothing. Don't buy anything that will be of no use. Don't even touch potions because you are still a beginner and you will not lose exp even if you die.

Level 11-20: When you first become a thief, you will be given a Beginner's claw (which is really an untradeable Garnier), an untradeable Fruit Knife, and 3 sets of non-rechargeable Beginner's Subi throwing stars.

Level 20-30: When you hit level 20, (in GMS), you can obtain a box via Tot's Know-How that contains equipment for your job and level. In other versions, you should stick with your old equipments, since it is most likely you will have no problem killing anything. As for potions, you should use Mana Elixirs and Unagis depending on your MP and HP.

Level 30: Now you're an official assassin. Congrats! Start buying mana elixirs as a replacement of blue pots (don't use pure water until your maxMP is above 1000). Also, by doing a quest that requires you to kill 99 Evil Eyes gives you 100 free mana elixirs. Use them when needed. Stick with white pots for now. Train, and get mesos. If you have Mokbis, which you should, sell them off and buy Kumbis/Icicles or Tobis if you have the money. You should have at least 2400-2500 total stars, which is 3-5 sets of whatever star you choose. Also, get all of the level 30 gear and do the Nella Kerning City's request 2 to get your level 30 shoes.

Level 30-35: After some time of training, you should have quite the amount of mesos. Get a lot of mana elixers and white pots. Buy the knucklevest set. Now that you probably know a lot about the stuff you need, get the stars you wish. Make sure to keep the strength arrangement as I told you before. Get any hat as you wish, consider the burglar because this will take sometime to level. Play your cards right, don’t waste mesos. You'll be needing more mesos to spend later. Try to keep up to around 500,000 mesos or higher. If you have the money, buy +7 guards, they will help you out.

level 36-40: You might want to buy the avarice at this stage or just keep the meba if its +7, and you may be able to afford some tobis. If not, continue to buy more kumbis unless you’ve had a stroke of luck in quests that your able to fund yourself with scrolls, in which case try to buy a set of tobis. The order that stars should be in your inventory is strongest to weakest, and here are all of the stars in that order: Crystal Ilbi, Ilbi=Hwabi, Steely, Tobi, Icy=, Kumbi, Paper Planes, Tops=, Mokbi, Wolbi=, Snowballs, Subi. Ilbi and Hwabi have the same attack power and capacity, but Ilbi are far more common (and you can equip them 20 levels sooner).

Level 41-50: Go to Ludi pq you can get some nice juicy exp and great items there. If you constantly do it and end up with a lot of money try to scroll your lvl 50 weapon (Slain) also try to upgrade slain to dark. If you do that and scroll it you can keep it until 70.

Level 51-60: if your looking for money go sakura massacring at midnight (otherwise it’s too crowded) for some very decent lvl 50 drops and about 350 mesos each. (just 2 sakura cellies will give you enough for a mana pot.) If you’re somewhat rich or funded buy your lvl 70 stuff ahead. Why not the 60 armor? Simple, the lvl 55 quest "Pia's request" will give you a free lvl 60 top and bottom. Stick to Tobis and try to buy another set. Also, you get the level 60 hat from the "Cleaning up the streets" quest in New Leaf City.

Level 61-70: Okay now its time to get rich in stars. Go at lvl 60-63 or so and fight red drakes, even though their exp to health ratio isn’t the best they drop something almost every sin craves: Steelys. At lvl 64 if you're armed with steelys go zombie hunting for quick exp and money. Grab the lvl 70 stuff now if you didn't before. Try to save up for a much needed Scarab in the next few levels.

Hermit Armor/Weapon Guide

Level 71-80: At this point you should have illbis or atleast 5 sets of steelie. At this lvl you should have or get a 6+ atk Wg. Stay with your Dark/White Identity and get a 40+ atk Scarab and scroll it. After all that go and train at Wind Raiders to make some fast leveling and good exp. Try not to buy any other equips at this point, because you will need it for future claws, which are very expensive.
Level 81-90: If you got your 90 stuff before, great, if not... well go buy full red osfa (best bonus) including the hat. Stay with the Scarab, its' luck bonus will do wonders while Black Mamba and Casters are overpriced plus Zakum exclusive.

Level 91-100: Stick with the 90 stuff, the 100 gear is only available from Pianus and at this level he'll be way too hard to fight. Start saving up for the Craven and prepare to sell your Scarab. Keep your money high around these levels, you can go broke fast. At level 91~92, if you have the money it's definitely worth buying some Casters with a weapon attack of 52+, especially if your scarab is below 50 weapon attack. A 10 attack glove is a must at this level, otherwise you'd be out damaged by hermits 2 levels.
Level 101-110: Now go get the 100 armor. Get the Garina stuff or stick with Osfa just don't use the katinas but you can use the level 100 gloves and boots if you can find them.

Level 111-120: Use Garina and take the purple dragon sleeve. Congratulations on becoming a Night Lord!

Alternative Assassin armor/weapon guide

This guide is for a funded assassin or one who has a lot of money by level 25-30 please note that this version is much more offensive so stationary bosses with area affect skills (zakum, pianus) with probably be to hard to take on until level 100. Overall this path is quicker to level with. (By the time you start here you should have tobis)

level 30-34: Buy all luck bonus armor and get the gloves fully upgraded or stick with dark wolfskin. Buy snowshoes and scroll it 7 times with jump 100% scrolls.

level 35-39: Buy the green knucklevest and blue pants get the arbion from john's present quest. Ludi pq all the way until 40.

level 40-49: Buy 5 luk cleave and dark shadow and buy 2 luk pants get an avarice that has at least 23 atk and 3 luk. Get a dark or red pilfer that gives +3 luk if you have bought all the other stuff.

level 50-59: Buy a maximum powered dark slain and a +4 luk sonata buy pants that give at least +3 luk then you have to save save save! (Buy steelies during this time period if you have enough).

level 60-69: Buy a 6 luk Identity that’s probably all you have money for now anyways. (get steelies if affordable).

Chief Bandit Armor/Weapon Guide

Level 70: Definitely keep your Silver or Dark Identity, as both have the same or more luk than the Bloodfox respectively. Get the Dark armor set, as they give the maximum amount of luk, and get a decent level 70 weapon, with at least 3 luk and 83 weapon attack, or if you have a low meso amount, stick with your deadly fin, although training will be slower, and there will be a big difference in attack.

Level 80: If you have a Dark Pirate Hat with more than 4 luk, then switch hats. If you don't, stick with your Dark Identity. Change your entire armor set, and consider getting a 10-15 attack work glove if your wallet is full. Scroll your level 80 weapon to +4 or more (93 weapon attack and 4 luk). If you have money for an ATT scrolled shield, it would be a good investment, but luk bandits may be lacking funds to buy shields due to unscrolled ATT shields having STR requirements.

Note:

If you have less than 20 million, you could get a workglove and scroll it with 100%s.
If you have 20 million to 50 million, I wouldn't recommend one, as unless you use your entire budget, you could probably get a 6-8 attack one.
If you have 50 million to 100 million, buy a couple of 60% glove attacks, make a work glove and scroll it yourself, although it will slice a clean cut off your budget.
If you have 100 million+, then get a workglove with at least 12 attack.

Level 90: Get the Dark Set again, and get the level 90 dagger (+ 4). You should have a workglove by now, and scrolled capes and shoes. If you don't want to spend your mesos on the top and bottom, at least get 100% scrolls or cheap 60% scrolls and scroll your armor, unless you're planning to sell the level 90 armor off.