Cheats:

 Type them in whenever you need them:

 idbehold        Displays menu (followed by S, V, I, R, A, or L
                 for choice)
                        S=Strength (Berserk)
                        V=Invulnerability
                        I=Partial invisibility
                        A=Full Automap (computer map)
                        R=Anti-radiation suit
                        L=Light amplification visors
 idchoppers      Gives you the chain saw
 idclev          Warp (followed by episode number and level number)
 iddqd           Degreelessness mode (God mode)
 iddt            Toggles Automap between normal, full, and full with
                 objects (enter when in Automap mode)
 idfa            Gives full ammo, 200% armor, and all weapons
 idkfa           Very Happy Ammo (full ammo, 200% armor, all
                 weapons & keys)
 idmypos         Displays your bearing and coordinates in hex
 idspispopd      No clipping (you can walk through walls)

 All cheats taken from the Official Doom FAQ by Hank Leukart