Clayton, you should start by completely reading through the
Event Information and
Novice Guide on the left hand side of the home page
http://www.omrscca.org/. There is a ton of great info there and it will answer so many of your questions. Some important points are:
-- Show up early, it will give you more time to learn what you are supposed to be doing and don't be afraid to ask what you can do to help. If you didn't already know this you will be required to work the opposite heat of which you run just as everyone else is.
-- Bring your Drivers License, a safe car, and preferably your own helmet. If you are under 18yrs old you must have both parents sign a minor release form and one parent must stay the duration of the event.
My car is stock other than the normal intake and exhaust.. I plan on lowering the car, and stiffening up the suspension and maybe putting in a 5 point harness at some point.
What is/is not allowed to be on your vehicle wile at an autocross event? What class would I run in?
Located in the
Cars and Rules is a link to the
2009_Solo_Rules.pdf so you can read the Category Sections to see what class your car would be in. It is ultimately the responsibility each competitor to classify their own car, but from your brief description I would think you should read over the Street Touring Category section, as long as you have wheels <7.5" wide and tires with a treadwear numerically higher than 140 (unless you have other mods than what you mentioned). I will give you the warning, be sure to pay attention to the rules before modifying your car because sometimes things that you would not think would be much of a performance advantage can bump you up several classes.
Don't worry about being to slow your first time out, everyone is slow the first time they try this. You don't need to make all of the improvements that you want to do with the car before the first event you come to, just make sure the car is safe. When you come to the first event be sure to listen to the advice given, try to get a ride along with someone in a similar car, and most importantly enjoy yourself. Don't get to hung up on how fast everyone else is going because if you finish the day going quicker you should consider that a success.
Hopefully you guys won't laugh to hard. 
