To enter the cheats, you need to press Left Shift and minus on the numpad at the same time and you can enter the following cheat codes:

-iwillbetheone
Enables God mode (you're unkillable) - This cheat code does not work on the steam version for some reason so beware.

-savegame
Saves your game (you can use this more than once, so infinite saves)

-campaign
Unlocks all of the campaign missions from the start.

-topography
Shows the map eleveations on the ingame map.

-endmission
Ends the current mission.

PS: When you hold left shift and numpad minus, nothing pops up, just type the cheat code and you're set!