Start game, goto console (hit tilde ~) and enter "LOGIN developer" without the quotes to enable cheatcodes. Then use the following codes:
FLY 0/1 - Toggle Flying Mode
GOD 0/1 - Toggle God Mode
NOCLIP 0/1 - Toggle Noclip Mode
UAMMO 0/1 - Toggle Unlimited Ammo
Note:
to be able to login as developer, you must enable remote access first, type "remoteadmin 1" without the quotes