Press ~ to display the console window, then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Demon mode - SV_DEMON_1
Level skip - SV_PERFECT_FINISH_1
Full ammunition - SV_MAX_AMMO_1
Full ammunition and health - SV_EVERYTHING_1
Full health - SV_MAX_HEALTH_1
God mode - SV_CHEAT_GODMODE_1
Keep decals - SV_KEEP_DECALS_1
Monsters always gib - SV_MONSTERS_ALWAYS_GIB_1
Monsters have less HP - SV_LESS_MONSTER_HP_1