Weapon Advice

1. Fist
Your marines fist is the weakest weapon in the game and should only be used in conjunction with a berserk pack, when you have no-ammo or when ammo conservation is important. The fist works best against weak enemies such as former humans and enemies that have to get close to attack (excluding Lost souls as they are too fast).
Best Vs: Former humans, Demons, Specters
Weak Vs: All other monsters

2. Chainsaw
The chainsaw is a decent close range weapon but like the fist, it's onlyuseful in low ammo situations. When using on demons, try to back up while doing it or you may get bitten.
Best Vs: Former human, Former Human Sergeants, Demons, Specters,
Cacodemons, Lost souls
Bad Vs: The rest.

3. Pistol
The pistol is a rather weak weapon but due to its range, it's more useful then the fist. Still, you should use it only when your last resort is the
fist. Once you have the chaingun this weapon becomes totally useless.
Best Vs: Human enemies
Weak Vs: Everything else

4. Shotgun
The shotgun is the best all around weapon as it is rather powerful, and ammo for it is plentiful. The one problem is that it has a slow reload rate so it's not good verses whole hoards of demons that are attacking from every direction.
Best Vs: Human enemies, imps, lost souls
Worst Vs: Cyberdemons, Spider Mastermind

5. Chaingun
The chaingun basically is a rapid firing machine gun that uses the same ammo as the pistol. However, the chaingun is MUCH better than the pistol and is useful for killing large amounts of enemies, although it takes a while to kill the enemies.
Best Vs: Human enemies, imps, cacodemons
Worst Vs: Cyberdemon, Spider Mastermind

6. Rocket launcher
The rocket launcher is the second most powerful weapon in the game. But you must be careful with it as the splash damage it causes as it can easily kill your marine. However this splash damage does lots of damage to the enemies so it evens out.
Best Vs: All enemies
Worst Vs: None

7. Plasma Rifle
The plasma rifle is a rapid firing weapon that does more damage then the chaingun. Be warned that some enemies seem to be able to resist the damage from this gun, and at times the projectiles can clog up the screen.
Best Vs: All enemies except Cyberdemons and Spider Masterminds.
Worst Vs: Cyberdemons & Spider Masterminds

8. BFG9000
The best weapon in both this game and probably any game I've ever played! Although it takes a while to charge and takes a lot of ammo, the damage dealt is massive. Best used against tough enemies.
Best Vs: All enemies
Worst Vs: Nothing, BWHAHAHAHA!

Fighting advice

General:

Learn to strafe. I cannot stress this enough. Strafing is the most useful ability in not only Doom but many other first person shooter games.

Know when to fight and when to run, if you have 2 bullets and are being attacked by a Baron of Hell, you should try to escape and gather some
ammo. Then you can return to finish him off (heh, heh, heh)

Try to sneak around and avoid fights. Monsters usually hear you when you fire off shots, so only shoot if necessary and keep as far away from monsters
as possible. Avoiding conflicts is probably the only way to survive on Nightmare mode.

Former Humans:
Use either the pistol or a close range weapon to take them out. No need to waste ammo.

Former Human Sergeants:
The sergeants are VERY annoying as they do a fair bit of damage with their shots. Try to take them out quickly with a shotgun, or a pistol if that's all you have.

Imps:
Are the most common enemy in Doom, so get used to them. You can tell if there are any imps by listening to what I call the clicking sound.
Anyway, the shotgun is the best weapon to kill them with. Usually you will have a shotgun by the time you first meet them, so fill 'em with lead and
keep moving.

Demons:
Pig like creatures that do a reasonable amount of damage, have a fair amount of health but must get close to attack. Best weapons to use are the chainsaw and probably the shotgun (if they are in small groups). The demons usually travel in small groups of 3 to 6, so weaken them with the shotgun then finish them off with the chainsaw, or even your bare fists.

Specters:
Exactly like the demons except they are invisible. Be especially careful in dark areas, where the Doom designers seem to like putting them.

Lost souls:
Flaming skulls that try to ram you. They usually travel alone or in large groups of 10 or more (on Ultra-Violence). Try to take them out quickly with either the shotgun or chaingun.

Cacodemon:
Huge flying 'balls' that shoot lightning balls, they are fairly common and can be easily killed by a chaingun, plasma rifle or chainsaw, although it does take a while.

Barons of hell:
Big goat-like demons, they deal lots of damage and can take lots of damage too. Try not to get too close to them as their scratch (punch?) attack is much more powerful then their slimeball attack. Use a powerful weapon such as the rocket launcher or plasma rifle to take these guys out quickly.

Spider Masterminds:
The final boss in Doom isn't really the hardest monster in the game. To take out these b#$%@!& use the BFG9000 3 times to finish
them.

Cyberdemon:
The boss at the end of the second episode, to kill the Cyberdemon, use the rocket launcher or BFG9000 while dodging his rockets.

Abbreviations and definitions:

N,S,E,W,NW,NE,SW,SE = North, South, etc. North is up on the map, east is right, etc.
Jump = Run off the edge of a platform.
Activate = Press the use (default is spacebar) button. If I say something can be activated it generally looks different then its surrounding walls.