Turn On Console:
Use Notepad or a similar text editor to edit the GameSettings.script file which is located in the Game directory.

Change

bool ConsoleEnabled = false;

to

bool ConsoleEnabled = true;

Now in the game press F3 to bring up the console and enter any cheat below

immortal = true Invincibility
god_mode = true God Mode
unlimited_ammo = true Unlimited Ammo
fullammo Max Ammo
enabled_missions = true Get All Missions