There are many ways in Combat Arms to customize your character and make it different from other people. There are many weapons to choose from and many different pieces of gear available to you. By clicking on the Shop/Storage and Modification tabs on top of the screen in the lobby or a game room, you will see all the things you have and all the things you can get.
In Combat Arms, renting items will cost you GP (Gear Points), which you can get simply by joining and playing games. The items can be rented for either 1 day, 7 days, 30 days, or 90 days, with the rental price increasing the longer you rent for. However, the unit per-day price will be significantly lower if you buy things for a longer time. For example, renting an AK47 for just one day will cost you 800 GP, but renting it for seven days will only cost around 3500 GP, which is much less than if you had 1-day rented the AK47 every day for seven days. You can customize your character is by renting weapons and/or bodygear.
Weapons
Everybody has four weapon slots, accessed in-game by pressing 1, 2, 3, and 4. Each of these weapon slots can only carry a certain type of weapon.
Primary Weapon
Your primary weapon is a powerful gun that you will very likely be using the most out of your four weapon slots. Types of primary weapons include Assault Rifles, Submachine Guns, Machine Guns, and Sniper Rifles. Assault Rifles are guns with a good balance of attributes. Submachine Guns sacrifice damage for rate of fire, machine guns sacrifice accuracy and portability for damage and huge ammo clips. Sniper rifles are powerful guns that are deadly from a range but very clumsy and useless at close range.
The default Primary Weapon for everybody is the M16 Assault Rifle. It costs 0 GP to maintain and stays permanently in your storage. However, it is not a great gun compared to the other Primary Weapons that you can rent for GP.
For information on the primary weapons, see Primary.
Secondary Weapon
The secondary weapon is a weaker gun that you can use in the case that you run out of ammo for your primary weapon, or find yourself in a tight spot. Types of secondary weapons include Pistols and Shotguns. Pistols are small guns that are only slightly weaker than assault rifles but are not capable of automatic fire. Shotguns are larger guns that are extremely deadly at a close range but almost useless at a farther range. Note that shotguns can be very powerful and act like primary weapons when in the hands of a skilled user.
The default Secondary Weapon is the M92FS Pistol. Like the M16 Rifle, it costs nothing to maintain, but isn't very great either.
For information on the secondary weapons, see Secondary.
Melee Weapon
The melee weapon is something that you can use to kill enemies at a range such that you are pretty much right next to them. All melee weapons have two attacks, done with the left and right mouse buttons. Melee weapons have infinite "ammo". Since you have to get close to an enemy to use the melee weapon, it is not used much. People will often use the knife to kill and humiliate people who are not watching their back (often snipers looking through their scope).
The default Melee Weapon is the M9 Knife. It isn't great, but since melee weapons are used very rarely anyways, most people do not bother to rent a better melee weapon with GP.
For information on melee weapons, see Melee.
Support Weapon
The support weapon is anything else that might help you in combat. Support weapons include a variety of explosives, including grenades and mines. Other things like Heartbeat Sensors are also available for the Support Weapon slot.
The default Support Weapon is the M67 Fragmentation Grenade. It is actually decently useful for killing enemies, and people that rent other support weapons usually place it in their Backpack (explained below) rather than replacing their M67 grenade with it.
For information on support weapons, see Support.
Modifications
By clicking on the Modifications tab from the lobby or game room, you can add some components to your guns to make them better. Modifications are rented for 1, 7, 30, or 90 days, just like everything else, but they are considerably less expensive. To add a Modification to a gun, simply select the gun you wish to modify from the menu on the left and then click "Attach". Not all Modifications can be put on all guns, as they are type-specific. For example, you can place the same scope on both an AK47 and an SCAR-L, because they are both Assault Rifles, but you can't put the same scope on a pistol. Some weapons like the G36E and K7 come with built-in modifications.
Suppressors
Suppressors go in the front of your gun and will decrease the volume of your gun shots, making you less likely to be spotted. The cost of this is reduced damage for your gun, but it most of the time barely noticeable. There are three different suppressors, all of which do a different additional third thing. The S1 Suppressor will reduce recoil, the S2 Suppressor reduces tracer, and the S3 Suppressor reduces muzzle flash.
Machine Guns and Shotguns can not be suppressed. The K7, SR25, USP Tactical, and L96A1 Ghillie have built-in suppressors which can not be replaced. All other guns can be fitted with one.
Scopes
Scopes go on top of the gun and will give you better aiming, accuracy, and/or magnification when used. To use any scope in-game, click the right mouse button. While using a scope, your crosshair is replaced with a dot or stationary cross that will not open up and become big like your crosshair will. Looking through a scope also reduces recoil.
Machine Guns and Shotguns cannot be fitted with a scope. All Sniper Rifles have a built-in scope that gives huge magnification. The G36E, AUG-A1, and P90 also have built-in scopes, with the latter's being a small low-magnification one. These built-in scopes can not be replaced. All other guns can be fitted with a rented scope.
Magazines
Different magazines can replace the default one that the gun has. Some magazine modifications will increase the maximum number of bullets that can be in one clip, at the cost of increased reload time. Other magazines will do the opposite: reduce the clip size but decrease reload time. Note that magazine mods do not change the total number of bullets, it only changes the clip size. For example, a normal AK47 will have an ammo count (see attributes of a weapon) of 30/90, while adding the Extended Magazine I will change it to 40/80. In each case, you still get 120 shots total.
Every single gun that exists can be fitted with magazine modifications, including Machine Guns and Shotguns. There are no exceptions.