1. Rebind the controls. Movement, jumping, crouching, using items. Alternate fire must be on a reliable mouse button - not the wheel. Mouse buttons are set up in the Setup, not the game.
2. Turn on the crosshair (I). And the mouselook (U).
3. Definitely rebind "Use item" from Enter. Otherwise on loading a save you'll automatically toggle the item, be it a health kit, jump boots or beast vision.
4. Look around the levels for secrets. The very first level alone has health, lots of armor and a friggin napalm launcher hidden so well, the authors of the guide video that came on OUHB disc 2 didn't find it. If a crack looks suspicious, blow it up (Dune Nukem rule). You can also jump on (and off) enemies' heads to get high.
5. Learn you way around zombies. Use the first fight to learn dodging. In general find places where the crowds can't reach you (they don't jump and can't open doors). Learn the growl they make when they're dead.
6. Flare gun is godlike. It kills a normal-sized humanoid enemy with 1-2 shots on any difficulty. Just learn patience.
8. If you light TNT's fuse, don't forget to throw it.
9. Save the Tesla cannon for the stone bare-arsed guys.
10. Life Leech's alternate fire turns it into an stationary automatic turret. It can fire all day and not drain your health.
11. If you got a huge crowd waiting for you at one entrance, see if you can find another one - the levels are not always linear.
12. Don't panic. Save often.
13. There's a secret level in each episode. It takes a concentrated effort to find but is worth the effort.

Aside from the technical content of 1 & 2, your "tips" border on being spoilers.

Describing guns vs monster efficiency is spoilers, considering that all weapons, items and enemies including bosses are listed in the manual? A suggestion to explore is "spoilers"?

good tip about the mouselook

I´m finishing the Ep3 without it, I will use in the ep4

Spelling out effective use of each weapon vs. letting the player discover the tactical advantages of each on their own, yes I'd consider that a spoiler.

Also, listing out exactly what you can find in secret locations, is kinda like leaving your Christmas shopping receipts posted on the kitchen message board. Granted you just listed it for E1M1, but you could be a little more vague. Saying something like, "Check for secrets! The game is littered with them and they are well worth the effort." would be enough.

I know you're just trying to be a nice guy and I've done similar things in the past, but I wouldn't want to inadvertently ruin it for a new user.

Come on, I mentioned 5 weapons out of 10. Alt fire of 2 out of 9. Granted, this isn't Call of Duty and the guns are more that "this shoots a bit faster".

As for the secrets...
OK, I'm open for rewrite suggestions. I won't cut out the flare gun advice. It looks like crap at first but in fact gets more lethal than the shotgun on Extra Crispy.