Great advice and posts so far guys!
As for tire pressure, it's the ONLY thing I changed when I took my car to the first AX. I ran 40 in the fronts and 36 in the rears. (you may need to run more as posted above) Get a bottle of white shoe polish and mark the edges of your tires so you can see how far over they roll. Make sure they get near or to the edge of the tread, but not past the edge. Rolling past the edge would have you losing traction.
I will also suggest walking the course. It is a good idea to show up early to the event and maybe walk it 2 or 3 times. (I walked it 4 myself) I went 2 times with a group of people. I listened to what they had to say, but remember, they could be wrong! I walked it the second two times trying to memorize the track and count how many R or L hand turns I would need to get out of the slaloms the quickest, and on one slalom was able to cut out an extra cone by entering different than where the track seems to want to push the car.
Listen VERY CLOSELY in the drivers meeting. A lot of good information can come out of it. (Though it seems to often be garbled in jokes and misc. info.)
If you get a chance, had a ride along in your car, or see if you can get a ride in someone elses car. You can learn a lot.
The event I went to was in the middle of nowhere, so you might want to bring a cooler with some drinks and food. There isn't always a lot of time to get to it and the day goes faster than you would think.
If you gt to shag cones in the morning, look for what might be a tricky spot on the course to work so you can see what others are doing wrong and you mmight not do the same. I wouldn't pick a slalom, they are actually rather simple to understand. It's not simple to drive, but they really aren't that confusing.
Definitely check out the link from the post above. You will learn a lot!
Admit to people that it is your first time and be polite to accept any advice they can give you. Everyone is usually helpful. (Though not always correct, so be careful)
If you are running stock, (no cai or other mods) you should be in GS. I had someone try to put me in another class with a worse multiplier, but I found the sheet (an official had it) to show my class.
Lastly, have fun! Your car kicks ass and, as I learned, is far more capable than I was my first time out!. I really hope to push it this weekend also, as I will be racing the 17th too. Enjoy the day and make some cool friends. Most all of the people there are friendly and helpful. Oh, and, your car will be gaulked at. You'll love that part.
PS: OH! almost forgot! Take a bottle of oil with you, just in case you start to run a little low. High RPMs and low oil = bad day.
Have fun!