Take a look at my thread for information on what the car needs to be really track-worthy:
ban25's Track Journal
I'm at a point now where I can run as long as I want on the track -- barring fuel -- and I've done sessions over an hour in length. Now I'm starting to seriously look into performance mods, but the very first mods you should do, in my opinion:
0 - Before Going to the Track:
- Clutch and Brake Fluid: Castrol SRF
1 - After the First Track Day:
- Track Brake Pads: Carbotech XP10 (what I use, but there are lots of good options)
- Performance Tires: Bridgestone Potenza RE-71R
- 34-row Oil Cooler (the largest you can get)
2 - After Several Track Days
- Suspension, Intake, Exhaust, etc.
First thing to change before going on the track is fluids -- you need brake and clutch fluid that won't boil under track conditions. I recommend Castrol SRF as it has the highest wet boiling point. Motul RBF 600 is decent, and I used to run it, but I've boiled it too and don't run anything but SRF anymore.
The next biggest upgrade is to replace the tires. I use Bridgestone RE-71Rs, the successor to the RE-11, and a tire with even better performance in my opinion. I've completed 7 track days on a set of these (19" 245/275), and I have a new set going on this Friday in time for a track day at Auto Club Speedway this weekend.
At about the same time, you're going to want an oil cooler. Before I installed one, I was burying the needle at 300F after only one or two laps in a session. Now, it's uncommon for me to hit 260F, even after a solid hour on track at WSIR. Make sure you get the biggest one you can buy, typically 34-row, with a thermostatic plate to ensure the oil can still easily get up to it's normal operating temperature (180F).
At this point, you have a car that runs reliably on the track and it's time to start thinking about performance. This would be a good time to upgrade the exhaust and instake, as well as get a tune (EcuTek). EcuTek RaceRom will help quite a bit on the track by offering a much less intrusive, tunable traction control that is usable on the track, as well as providing the ability to remap the throttle and improve the throttle response at lower gears and lower RPMs.