what to do

my girlfriend wants to know what she can do to her 1999 sentra 4 door automatic

Depends what you mean.
Cosmetic upgrades?
There are body kits, repaint, wings, window tint, all that kind of stuff.

Performance?
Again lots of choices.
Bigger wheels, brakes, upgrade suspension.
Intake, exhaust, header, cams, eengine program.

Again, it depends on what you want to do, and how much money you have.

ok, I know what can be done to a car, but I need to know where I can get the parts. I know what possibilities are, but I cant find any good web sites with parts.

Here are a few....

Try www.stillen.com
Started by Steve Millen, racer for Nissan. Click on sports cars, and then find Nissan.

You could also try....
www.nopi.com
Find their online catalog and look for Nissan.

Another would be
www.B15sentra.net
This is a community for Nissan B15(sentra) though I am not sure if a 1999 is a B15 or a B14.

Other places would be parts outlet type places like....

www.jegs.com
www.jcwhitney.com

but get involved in some of the communties on the net like B15, or find one for Nissan.

Most of them are great. Full of advice, group deals, help with problems, and even some step by step guides about how to do mods.

Also, check out www.nissanperformancemag.com
It is an online publication dedicated to modding Nissans.

Hope this helps.

I have an 02' Spec-v, so I know what you are looking at.
There is lots of aftermarket stuff if you have a Honda, or an Eclipse.
There are parts out there...

Oh yeah,
Also try....
mossy performance.
Their website seems to be down, but I got an absolutely beautiful cat-back exhaust system from them. It was my first mod and I still love it....

Good luck...