Teleglitch

Resource management is the name of the game: Save ammo unless death's staring you in the face. but don't overdo it or you'll die with a full backpack. Save health - avoid getting damaged, but do eat the meat to keep your HP up. You can unload spare copies of weapons by picking them up and dropping them. Learn when early weapons become dead-ends, and when you get better stuff just use them up and leave them.
Do the first 1-2 levels with the knife (back off to control your reach so you don't get mauled), it's there to help you conserve ammo. Also, enemies have some momentum, it's fun to kite hordes into black pudding.
You hold the knife in right hand, the fork gun in the left. This matters for accurate aiming and knife-fighting.
The map is your friend and a handy tool to tell where you haven't explored.
Scavenge everything, combine items, put useless parts into weapon upgrades and whatnot. Remember that you can stash extra stuff and come back for it until you exit the level.
Craft the lifesaver item (Teleporter; Medkit50 + 2 * Mchip or 2 * Medkit25 + 2 * Mchip) ASAP and carry at least one at all times. It'll save you ragequits.
The always-closed doors in boss arenas are there to respawn from if you use the above lifesaver and can't return to the level start for obvious reason.
If there's a crack in the wall that looks like a shitty texture joint, it may a secret place you can blow/shoot open (can take a few shots and may or may not be worth the ammo use).
if there's a [ ] around a cabinet or terminal, it's one you haven't seen/opened yet (even if you've read all the terminals, there are special ones that will mark exits/cabinets on the map).
Don't assume that current knowledge will apply to the next level, the game likes to surprise you. Usually with a swift kick in the balls.
Each starting level gives you different starting item loadout so try them out, but it's up to you whether it's worth losing the potential savings compared to starting over from the beginning.
If you craft a new item combination, it'll go into the database and be viewable from the main menu even after you die.
Some enemies like to feign death. Don't assume it's dead just because you shoot it and it's lying immobile on the ground. If it's bleeding/drops weapon, you're safe.