How do you customize guns on Call of Duty 4 Modern Warfare?

It depends - if you simply want to swap a weapon (whether in singleplayer or multiplayer) just walk over to a weapon on the floor and press F (or what ever key you assigned to the USE function). If, in multiplayer, you want a better weapon, you need to complete all of the singleplayer missions, and then on the Multiplayer menu, select Create a Class, and customise your perfect loadout. For example, if you were more of a sniper than an assaulter, you would have perhaps the Draganov (or other sniper rifle) and maybe a handgun or smaller gun. Also, you can select several 'perks', things like increased accuracy, or camouflage yourself. The possibilities are literally endless.