Giu's advice is actually pretty good - blunt, but good. Your skipper probably already knows you're a newby, and that you don't know how to do anything yet, so she'll have you do things that are within your skill level, like sit wherever she tells you, to use your weight as movable ballast, and move from one side of the boat to the other.
As a general rule, stay out of the cockpit unless you're told to be there. The only people who should usually be in the cockpit during a race are the skipper, tactician and pit man, unless it's a really big boat. Those are the people who need to be there, and others will get in their way. When it's time to take equipment out or put it away, such as folding sails, be ready to help. It'll be appreciated.
Watch what each person is doing, and how they do it. Ask other crew members to explain how and why they do what they're doing. A good skipper will look for opportunities to let you try different jobs when he thinks you're ready, while sailing out to the race course, for example.
Be willing to help and eager to learn, and, as Giu says, "shut the hell up and listen to what they tell you to do." You'll soon learn that racing a sailboat isn't primarily a physical sport - it's a cerebral sport. The skipper and tactician have a gazillion things to think about, and it requires intense concentration. Don't break their concentration by asking them questions during the race.
Many non-sailors have difficulty imagining how it can possibly be fun to race a boat that often doesn't go much faster than a man can walk, but you'll soon understand. Enjoy!