To enable cheats, while playing a game, bring down the console with [Shift + ~] and type: "devmapall". You can then enter any of the following codes in the console:

Cheat Code - Effect

god - God Mode
noclip - No Clipping Mode
notarget - Enemies Ignore You
kill - Kill Yourself
give all - All Weapons and Health/Armor at Max
give health - Health at 100%
give armor - Armor at 100%
give ammo - Ammo at 100%
give X # - Give Item X (# = amount)