Press tilde while playing to get your console, then type in the following cheat codes.

Effect	Code
All Weapons+Ammo	sv_everything_1
Demon Mode	sv_demon_1
Finish the level at 100%	sv_perfect_finish_1
God Mode	sv_cheat_godmode_1
Max ammo	sv_max_ammo_1
Max Health	sv_max_health_1