Get a tutor. No point waiting until you fall behind. Get a tutor from the beginning. There is no such thing as having too much help. A tutor can help you feel confident and reinforce what you learn in class so that you can keep up.
Read. No kidding, but you have to read the right stuff. Talk with the teacher about some readings outside of the textbook that could help round out your knowledge of American history. You'll never know all there is to know, but the more you can pick up, the better off you'll be when it comes to the AP exam.
Get organized. AP American History isn't just another high school course. You now have the potential to earn college credits before even being accepted to college. You don't want to blow this opportunity. Establish a notebook for class notes and a folder to keep all your reading materials organized.
Start a study group. Yes, there is strength in numbers. Chances are you aren't the only one in class who's overwhelmed by the amount of material you need to cover before the AP exam. Form a study group early in the year so that you can develop a study routine. Remember to invite some of the brainiacs to join. Most of them will take advantage of any opportunity to improve their knowledge of American history, and you might learn something from them.