This would probably have been better suited for the College Life forum. Just for future reference.
A good idea is to find a local college adviser and ask him or her about their Psychology department. The college counselor should know best what sorts of classes you'd need to take in order to prepare yourself for their college Psych program. Plus, if you decide to go there in a couple of years, you've made friends with the adviser already.
My less educated, "I took Psych classes in college" guess is that since Psychology is a science, bulk up on your math and science while in high school. If available, look into Advanced Placement classes your senior year, specifically AP Psych or other math and science courses. You get college credit for those classes by passing a test at the end of the school year, so you can get a jump start on your major while still in high school. And even if you don't do so well on the AP test (they are scored out of 5 and let's say you get a 2), at least you've studied the material already and can review it when you're in college.