Before you start worrying about what class you want to compete in, you should just get lots of practice and learn the skills you need. Nitrous usually isn't allowed for safety reasons, but there are few places on most tracks where you could make good use of it, anyway. (Sebring being a notable exception)

I'd recommend doing a few autocrosses first. They're cheap and safe and give you an opportunity to hone your driving skills and feel your car out a bit before heading out to a track where mistakes can be very costly. Check the sticky posts here for more info and a schedule of events.

After that, prep your car (needs to be able to run reliably under almost constant acceleration and braking for 20-30 minutes without overheating or other problems, needs good tires, needs good suspension, needs really good brakes with pads and fluid that can handle high temperatures) and sign up for a track event with any of the area track clubs. Most of those clubs will provide you with good instruction to keep you out of trouble on the track. (like Chin Motorsports, PBOC and NASA) SAFE Motorsports doesn't have instructors, but all you have to do is find a member of that club who's willing to "sponsor" you and act as your mentor for a day and you can get out on track with them.

After you've got many days of track experience, you'll have a better idea of how you want to proceed. Some of us are happy just to go drive fast around the track when we can. Others have that competition bug and want to go racing. Once you start gaining the skills, and talking to people at the track, you'll be better able to make an informed decision about what class to race in should you decide to go racing.

Hit us up with any more specific questions you might have after you read the stickies and check out some of the club websites.