Press '~' to bring up the console and enter any of these cheats:

Code - Effect

Cheat.Heal() - Brings health back to 100
Cheat.GiveAmmo() - Full ammo
Cheat.GiveRifle() - Gives a rifle
Cheat.GiveDynamite() - Gives dynamite
Cheat.God(0) - God mode off
Cheat.God(1) - God mode on
Cheat.MagicAmmo(0) - Unlimited ammo and no reloading off
Cheat.MagicAmmo(1) - Unlimited ammo and no reloading on