If you're going to survive Wolfenstein 3D, you'll need more than a quick trigger finger.  Here are some tips to help you escape the Castle:

Don't Rush Into a Room!
Let the Nazi guards come out at you.  Peek your head in a room, back up quick and often they'll be nice enough to open the door for you.

Conserve Your Ammo
Although it's fun to just let the bullets fly, you should try to restrain yourself.  Use short bursts with the machine guns and make every bullet count.

Secret Passages
Hidden rooms can be located almost anywhere.  Most of the entrances are marked with such items as:  the blue puddle, the pile of bones, the earthen pot, the green ceiling light, or a wall block that is out of the ordinary.  To find the ones that aren't, make sure the room you wish to search is free of Nazi rubble first.  Then hold down the Space bar (or button on a GamePad) and strafe across the wall.

Charge!
If you run at someone and shoot them point blank [rather than from a distance], you are more likely to wound them.  Plus, this may provide less time for them to shoot you.  However, killing is a two-way street.  These Nazis have a habit of appearing from out of nowhere and shootin' ya in the back!

Need Some Help?
Out of ammo, beaten to a pulp, and want to survive?  Hmm.  Try typing LEDOUX.  Beware, cheaters never prosper!