Press ~ (tilde) to bring up the console window and use the codes below.
Code .... Result xgod.... God Mode xfly.... Fly Mode xghost.... No Clipping xwalk.... No Clipping/Fly Mode Off killpawns.... Kill All Enemies setjumpz #.... Set Jump Height (500 = Normal) setspeed #.... Set Speed (2 = Normal) summon #.... Summon Item allammo.... Unlimited ammo