In game Press Key '9' to access console then enter following cheats.
Code - Effect
mpammo - Restore ammo to in-hand weapon
mpclip - Spectator mode
mpgod - God mode
mpguns - Get all guns & full ammo
mphealth - Full health
mppoltergeist - Ghost mode
Stand knocked over tree back up
In the first level the tree behind where the tank crashes can be knocked over and stood back up. Do this by pressing the "activate" buttons while pointing at the trunk of the tree.