To enable cheats, press L to bring up the console. Then delete the /tell command (do this each time you enter a cheat) and type: /cheatcheat, then press ENTER. Then enter one of the codes below to activate it:

Flight mode /fly
God mode, unlimited ammo, flight mode: /ac
God mode: /godmode
Invisibility /invis
Lose mission: /losemission
Toggle enemy field of view: /fovs
Unlimited ammo: /ammo
Win mission: /winmission