Take an AP computer science course through high school or independent study. AP classes help prepare you for the exam, but the College Board also offers study materials appropriate for independent study.
Practice computer and problem-solving skills with the free-response questions available at the College Board website. Download either Computer Science A or Computer Science AB to your computer. You must view these with Adobe Reader, which is a free download at the Adobe site.
Understand that the AP Computer Science exams center around solving problems through developing computer programs. Computer Science A covers object-oriented programming methods taught in a first semester AP computer science course. If you've taken more than one semester of Computer Science, look into taking the Computer Science AB exam that covers similar topics more in depth and includes algorithms and data abstractions.
Look into other study review tools. You can purchase monster-size review books at the bookstore or online, but there are free options as well, such as the College Board's free response items.
Attend Advanced Placement reviews offered by qualified people. Ask for referrals at your school.