Press ~ (or the key above Tab) to open the console. Type cht_bEnableCheats=1. 
Press ~ , Esc, [F1] to open the Cheat Menu (Max Health, All Guns, Level Select etc.)
At the bottom of the Menu the Cheats option will now appear. 

Go to this and type cht_bEnableCheats=2 for more options like God Mode and 
invisibility.