Go to the cheats screen from the main menu and click on keyboard. Then simply type the cheats in. Make sure you uncapitalize the first letter because it automatically capitalizes it for you.
iamgod - Enemies can't hit you.
idontreload - Infinite ammo and you don't need to reload.
Iwhanttokill - One-shot kills.
iamfast - Run five times faster.
iwhanteverything - Turns on all cheats.
iaminvisible - You become invisible.
iwhantmoney - You get $3000000000.
iwhantitems - You get all the items.
iwhantweapons - You get all the weapons.