Pre registration will help you know what time of day you will be competing. Your best bet is to ask someone what to do. In general, you will need to find the registration table and the tech inspection area. You will be given a number to display on your car. Ask if they have a novice program, and if so, enroll in it.
Autocross events are friendly gatherings and are open to all.
Feel free to ask questions. Autocrossers generally enjoy giving advice to newcomers. Most of all, don't get frustrated. Novices often finish low in their first several events. Experience will improve your times and placement.