Try enabling the following effects in the game to help you progress in the game by typing the codes during gameplay.

Unlock God Mode:
Iddqd

Unlock Temporary Invisibility:
Type in the code idbeholdi

Unlock Temporary Invulnerability:
Type in the code idbeholdv

Unlock ALL Weapons, Max Ammo, 200% Armor and Keys:
Type in the code idkfa

Unlock ALL Weapons, Max Ammo and 200% Armor:
Type in the code idfa

Unlock Automap:
Type in the code idbeholda

1x: Show Entire Map; 2x: Show Enemies:
Type in the code iddt

Eliminate ALL Enemies on Current Map:
Type in the code fhhall

Display Current Location:
Type in the code idmypos

Unlock Beserker Pack:
Type in the code idbeholds

Unlock Chainsaw:
Type in the code idchoppers

Unlock Light Amplification Goggles:
Type in the code idbeholdl

Player(s) Ignored by Monsters unless they are injured or a shot is fired:
Type in the code fhshh

Plays music from Episode #, Mission # (Ultimate Doom) / Plays music from Level ## (Doom II and Final Doom):
Type in the code idmus##

Go to level ## (Doom II and Final Doom) / Go to Episode # Level # (Ultimate Doom):
Type in the code idclev##

No Clipping (Doom II and Final Doom):
Type in the code idclip

No Clipping (Ultimate Doom Only):
Type in the code idspispopd