Welcome To autocross.
The best place to learn is in the stock classes, but your 16 bump you out of that. the next best place would probably be STS (Like I run). Classing is not really an issue right now because you've never done this before. Most likely you'll be placed in the N classes (novice) if your region has a novice program (Mine doesnt).
as far as preping You and the car, spend more time Preping You rather then the car. (Because any extra prep you do to the car isnt going to help you now anyways).
Start out by praticing proper hand position on the wheel.
1. 2 hand on the wheel! Allways!!!! the only time your hands leave the wheel is to shift, then its straight back to the wheel. (Dont rest your hand on the shift knob).
2. Hands at 3 and 9 (10 and 2 like your driving instructor tought you dosnt give you enough leverage on the wheel).
Second is Proper seating position:
3. First, your seat should be adjusted so that you are able to depress the clutch completely and perform heel/toe braking without banging your knee on the steering column. You should also be able to use the dead pedal for support through high-g turns. Your shoulder harness should be tight so that you do not need to rely upon the steering wheel for support.
Next, you should have a 90- to 120-degree bend in your elbows with your hands in the proper 3 and 9 o'clock positions on the steering wheel. This will allow you to make a 180-degree turn of the wheel when you need to correct an oversteer slide.
Finally, your butt should be tucked firmly into the seat back so that you can get feedback from the chassis and up through the seat. This will give you a better feel as to what the rear end of the car is doing. In keeping with the Zen philosophy of racing, you should become one with your car. Get comfortable, go faster.
Third, Learn to be smooth with the controls
meaning dont pounce on the brake or the gas, dont whip the wheel around and dont slam it in to gear and drop the clutch.... but remember Being smooth does not mean being slow. it just means that you are giving the car enough time to react to what you are telling it to do.
Fourth and Very important, Look ahead!!!!!!!
Drive by looking, look where you want to put the car on the corner exit and you will put the car there. Simple and smooth. Keep your head up and looking to where you are going, not just at whats in front of the hood. ANd NEVER EVER look in you mirriors to see if you hit a cone.
There is tons more, but that would just confuse you. Right now you just need seat time, so go out and Drive. Dont worry about being faster or slower then any body, as long as you are geting faster then you use to be.
For car prep, leave the thing as it sits now. Dont put another dime in to it or change a part. None of that is going to help you now any way. This is one of the few autosports where throwing money at the car will not nessisairly make it faster.
now on to some tips.
1. your going to need to run your tires at a higher pressuer then you would normally run on the street. when I had 17s I ran the fronts at 45-50 and the rears at 35-40 depending on the track. Pick up an air tank if you can and a good pressure gauge.
2. Bring Water, sunscreen, a jacket ect...ect...ect..... autocross is a run/work sport. so if you race, then you are going to work the track too. and standing on the asphalt chasing cones is no fun if your thersty and burnt.
3. check all your fluids the day before the event. and empty out the car of any thing not bolted down. (Spare tire, jack, subwoofers, any and all free floating objects ect....)
4. IF you have a helmet and want to use it, it has to be Snell 98 or newer (Most regions have loaner helmets)
5. Talk to every one and any one who looks like they know what they are doing.
6. walk the course untill they kick you off of it..... Preferably with a more experianced driver.
7.Have fun cause thats the point really.
Links:
Solo II car classifcation and rules Autocross.com a site dedicated to autocross Racer Wholsale, good place for helmets and other safty equipment I reccomend getting this if you dont have a Harness Physics of Racing Some tips from the pros Novice handbook 
