Well, there isn't much to say about a "getting start" guide but I'll try to do something here.
Something you need to know about the game is that any class can use any item in game. That means a sorcerer can use swords not as much as the warrior can use a staff. Combination of items can help a lot, however, remember: each class has it's specific item and, e.g., using a bow on a sorcerer will not be as effective as on a rogue. Even spells can be used by any class.
Your character must have some "things" to go on through the levels of the abyss, otherwise it will get pretty difficult to you.
- a Town Portal spell or scroll it's almost your life! You must have it, no matter what! It can save your life on difficult times, or even favour you if you died and created a portal before it happens.
- potions are very useful, don't bother to buy a whole inventory of it 'cause it's important to have those, since it replenish your mana or life pools. They can be very expensive at first, but you'll notice that almost everytime they're dropped by the enemies.
- some scrolls may hold important magics that only sorcerers can cast. If you're a sorcerer and find one of those, save it till you find some demon pretty tough and use it mercilessly.
Well, I'll give a fast explanation about the attributes in game so you get familiarize with them.
- Strength: it's your physical power. It will influence on how much damage you deal on enemies; and it's almost the main attribute that high-level equipments require. To see how much damage you can deal, check your damage indicator on your character stats tab. The numbers shown are the lower damage you can deal and the higher (e.g.: 13-50).
- Magic: your spiritual power. It will influence on how higher spells you can learn, and how much damage they can deal. However, magical damage can be increased too if you read the same spellbook more than one time, increasing the spell level. Increasing magic attribute will also raise the capacity of your mana pool. To see how much damage a spell can deal, go to yout spellbook tab and simply pass with your mouse on it, it will show the damage you can deal the same way that melee weapons.
- Dexterity: your capacity of dealing damage. It's an attribute that rogues must increase, since it helps your shots beeing more accurate. Increasing your dexterity will also increase your chance to hit and your armour class (both checkable on yout character stats tab).
- Vitality: basically your life. This attribute will only increase your life pool and that's pretty much it. Tip: Sorcerers should try to increase this attribute a lot, since their life pool are pretty low.
Something really important to emphasize it's the character's resists. They are pretty good to increase, however, it can only be increased through items. Resists will make your character less vulnerable to certain elements (such as fire, magic and lightning). Tip: if you play on Nightmare or Hell mode, your character's resists will be negative, and that will make the game more difficult. So, on those difficulty levels, try to find some item that raises your resistances a lot, may be helpful.