Autocross is a great all around sport. You get to compete in a safe venue, learn about driving your car near its limits, and meet lots of interesting people.
You will need to empty out your car of all loose objects -- so bring along a tarp or a box or something to keep it all together.
Make sure that you know what equipment you may need to bring; you will probably need to increase your tire pressure.
ACCO provide loaner helmets but some clubs do not.
Make sure that your car is in sound shape. Most autocross clubs do a tech inspection; they don't like to let cars out on course with loose batteries, loose wheel bearings, or bad ball joints (to name a couple of things that might be checked.) I have had to reject cars in tech that had incredibly wobbly rear wheel bearings, and once I inspected the car of someone who wanted to drive an autocross on tires that were showing cord.
Get to the event reasonably early. Registration often opens up quite some time before the main event starts. Early arrival will give you time to get oriented, walk the course, visit the little autocrossers room, and possibly help out the event organizers (this latter is always appreciated.) In particular, by helping out early on, you may get to listen in on course design in progress, and thereby learn something.
A stint in tech inspection is good for a novice autocrosser; you can learn something about cars in the process, and learn about things that you need to keep an eye on.
ACCO events requires that drivers also work stints in various jobs. If you want to be a well rounded autocrosser, you should try and learn all of them. Tasks include timing & scoring, starting, and corner working (aka cone shagging.) The heaviest requirements are in cone shagging, so expect to become expert in this first.
ACCO alternate runs; you drive, then you work, then you get a little time off, then you drive, then you work, and so forth. There are two important issues here: don't blow off your assignment, because we really do need everybody, and report promptly. After your run, it's ok to take a minute to check your tires and drink some water, but don't get caught up socializing -- because somebody is out there shagging cones who probably needs to prepare for his next run, and he can't do it because you haven't reported yet. ACCO will take your times away if you miss work assignments; I know it seems extreme, but if they're doing that, then probably they had an abuse problem sometime in the past.
You will spend some time standing out there watching the cones; everybody does. It's not wasted time; take advantage of it. You can watch the different drivers and start to see what some are doing that makes them consistent winners; look at the lines they take through corners, look at how they brake and when they get on the gas. But don't forget your job: when somebody takes out a cone, you need to restore it before the next car comes along.
This is not so important that you should risk your life; the basic rule is always that you should never turn your back on a moving car. Wait for a safe interval, go out and check the cone, put it back in the box if necessary, and if there's a penalty, signal timing and scoring. Signals may vary, so I won't go into them here.