I agree with all that has been said above ^. Start out with AX and DE and work up from there. Drive your completely stock car for the first year - the only upgrades you may need are a set of performance street tires and a better set of brake pads - but only after you've worn out the stock ones.
Once you know what you're doing on the track and are near the top of your stock class, then consider moving up to time trials. Once you get there, you may want to consider modifying your car to the Boxster BSX level. This is primarily upgrading the suspension to improve track performance but I also added racing seats and harnesses. You'll also need the Boxster roll bar extension. The car will remain street legal. Then drive your BSX until you are near the top of the BSX class.
The key to all of this is seat time, seat time, seat time. You will find yourself learning about cornering techniques, braking techniques, throttle control, tire management, and just how big your hairy ones are.
And while you're getting that seat time, I found that getting some professional instruction along the way is priceless.
At the BSX level, you may also want to start investing in safety equipment because you'll be driving on high speed tracks and the dangers are there. Something to consider is a 3-layer racing suit, gloves, shoes, undershirt, and a HANS device.
If you go this far and are ready to go wheel to wheel racing, then you'll need to strip the car, install a cage, and add all of the safety equipment. My Spec Boxster will remain street legal so I can drive it to/from events but its not really meant to be a street car (no heat, no a/c, no radio, no side windows, stiff springs, etc). Here is the link to my build:
Spec Boxster Build
The best way to start is to start simple: Get a helmet and start doing a few AX's and DE's. Don't get too far ahead of yourself, it will all become clear as you progress. And before you know it, you'll be selling your furniture to buy new tires.
