Factory Service Manual... hahahaha, j/k. But seriously, if you are gonna start working on your car, get one. You can "search" and d/l pretty decent versions on here.
I'm was in the same boat as you, 89 240sx coupe. First 2-3 years I spent thousands just fixing it to get it to perform stock. This last year it's been good (replaced damn near everything, so it better run). I've done basic things (intake, header, hi-flow cat, exhaust, suspension, etc) and now off to doing bigger things (turbo).
First off, you need to decide where you want to take this car, what you plan on budgetting, how much work you want to do yourself/how much you want to pay. Whether you are all show, all go, both.
WARNING: it's addicting, like a drug. Once you start, you are gonna crave doing more.
1st: know your car. know where things are located. the more you know the better you'll be off.
I would suggest going on
http://www.240sx.org/ - they have lots of info on there from maintenance to performance.
http://www.nissanperformancemag.com/
http://www.sportcompactcarweb.com/
SCC has a silvia project car. They go through modding it. Read it, it's a good idea of what you can do w/ your baby.
on here, you can search for things like "basic modding", "performance modifications", or something along those lines. This question has been answered so many times in so many different ways.
it feels a little shaky when you drive: have you checked your tires? gotten a balance and rotation? gotten it aligned? properly inflated? those are the common problems.
Ways to improve performance (a small preview):
1) Intake/Header/Hi-Flow or Straight pipe/Exhaust for starters
2) Suspension, wheels, strut bars (more showy, but some people "feel" a diff), sway bars
3) Strip your interior, remove your seats, lightening vehicle - getto but free
You will probably be overwhelmed at first, but don't worry, it will all come together, especially w/ the help of such cool people on this board (and a few ***holes who will just say things like "search button", but they are cool most of the time, haha).
BIGGEST TIP: you should check out some so cal meets . plenty of ppl you can talk to about cars and such in person. There's one going down October 9th near Puente Hills mall, I should be there, will be my first time to a meet.
Also, if you want to chat about cars, girls, games, or whatever, just hit me up on aim: agentsonic02 <- that's a zero and a two.
Good luck, hope you the best.
p.s. whatever you set your "budget" at, prepare to toss that out the window once you feel your first mod.
I suggest something simple like a Cold Air Intake. Injen is the only one who makes it for 89, carb legal and all. $200. Or you can read 240sx.org on a home made CAI for like $30-$40.