First, if this is your first event, let someone know. Many regions have programs for novice drivers, and already have everything you need waiting for you. You should also do a quick once-over on your car. Be sure to check the fluids, check the battery tie down, shake the wheels and remove any lose items. You will need to check in, and have your car inspected at "tech," both of these should be well marked and easy to find. Finally, make sure you leave enough time to get a few course walks in. There are no practice sessions on autocross, so walking a course is crucial in being ready to do your best lap.