Since it sounds like you haven't done much prep time, my best advice is: Eat a big breakfast & pack snacks. It will be so busy, the odds are slim to none you'll be able to get into any of the sit-down restaurants, and wait times for counter-service food will be as long as lines for many of the rides. Depending on which park you're visiting, you may even be better off leaving the park for one of the resorts. Their sit-down restaurants will probably be booked solid, too, but the wait at the food courts might not be as bad.
Otherwise, as has been said, go early, much earlier than the official opening time, do as much as you can in those first few hours, and resign yourself to shows, smaller attractions and people-watching as the day progresses.
