Here's the description of the AP music theory curriculum and exam.
http://media.collegeboard.com/digitalServices/pdf/ap/ap-music-theory-course-description.pdf
If you aren't from the United States and aren't familiar with them, AP courses are taken at the high school level by very advanced students and are supposed to be as difficult as a first year university course. At the end of the year, you take the official AP exam in the subject and a high grade will often count towards your college courses when you get to university. Often, passing the exam in high school exempts you from an introductory course at the university level. Familiarizing yourself with all the subjects and skills here is definitely a good start! Finding the teachers edition (so you have the answers to all the exercises) of a textbook used for such a course might be helpful. There are also commercial study guides and practice tests available that you can buy. There may be an ear test, as well as a paper test, where you will need to identify relationships between notes or types of chords by hearing them, rather than seeing them.
Other than music theory, music programs generally want you to audition on your primary instrument. If you are interested in being a composer or to study music history, rather than becoming a performer, the audition is less critically important, but it still matters. How much would you believe someone who says they want to become a composer, if they never learned to play an instrument? If you take music lessons, be sure to tell your teacher you are planning on music school, so they can push you to practice. You'll need a piece or two that you play VERY well to audition with. There may also be a sight reading test.
Learning a little bit of music history is a good idea too. Listen to the works of famous composers so that you can recognize the most famous ones when you hear them and learn a little bit about when they were written. You don't need to know every detail about everything, but the most interesting ones are good to be at least a little familiar. Mozart died in the middle of writing which piece? That sort of thing.