General Novice Advice
First, if you're a first-timer, be sure to read the novice guide linked to in the first post of this thread. You can search around and find lots of similar guides, but they all say about the same stuff and that one is well written and thorough. It's a good idea to check the website of the specific club you will be autocrossing with, as well. You may find some information on how THEY do things that could be very helpful to you.
Next, understand that with very few exceptions, autocrosses are run by car clubs and staffed by volunteers. Nobody is getting paid to do all of the planning, organizing and setup involved in putting on an autocross, and there's more to it than you might think. So, try to respect the fact that these people have volunteered their time so that you can have a good time!
How can you help? As a novice, the best thing you can do is try to show up early and simply ask "how can I help?" If you can't make it out early, try to arrange to stay late to help clean up. After you've been autocrossing for a while, you might try to learn some of the more specific critical tasks. For instance, you might ask "how does that timing equipment work, and can I help you set it up so that I might be able to set it up FOR you next time?" The more you learn, the more involved you can be. The more involved you are, the more fun you will have!