As I just finished AP US, I can definitely see where you are coming from to want to do your best to prepare. Throughout the year, you will be most concentrated on the time period from about 1800-1950, so I would begin to read up on those time periods. Go out and invest in an AP US book - i would suggest the Kaplen one - it gives chapter self quizzes and really helps analyze each time period. Begin the read through the beginning at least, and you will be ahead of the game in your class. One thing I can definitely recommend if just find article written during different time periods, and watch the history channel! A lot of it is actually really interesting, and you will learn a ton!