Full Ammo
On the second level on campaign mode when your officer shouts go go go! 
Immediately type in moreammo and you will get full ammo.

Various Cheats
At the indicated screen, hold L-Shift and press Keypad Minus. Then type 
any of the following "Codes": 

Code            Effect 
savegame      Saves Game (Gameplay) 
endmission    End Mission (Gameplay) 
campaign       Unlock Levels (Main Menu)

God Mode
At the indicated screen, hold L-Shift and press Keypad Minus. Then type
any of the following Code:
Iwillbetheone