It’s great that you have already identified a college major that’s of interest to you – congrats! In order to best prepare for a biomedical engineering degree, I’d recommend taking a college-level prep course of any or all of the following: geometry, trigonometry, calculus, physics, chemistry, biology and anatomy & physiology. If your school offers advance placement courses (which can sometimes count as college credits), then it would definitely be worthwhile to take AP Biology & AP Calculus. If AP Physics is available, that will surely help as well. Most engineering universities are looking for well-rounded individuals, so while focusing on your math & science is good preparation for an engineering major, don’t completely neglect your humanities or arts classes. If your school offers classes in Computer-Aided Design (CAD) I would strongly recommend that you check that out. If they don’t offer it you can always download a free version, like Google SketchUp, in order to practice on your own at home.
If you are eyeing a specific college, you may also want to check out their academic requirements for admissions. For example, many colleges will require 4 years of math (including pre-calculus), 4 years of English, and at least 2 years of a lab science. Your high school guidance counselor &/or the Math & Science Department heads would be able to help you figure out which are the best classes that are offered at your school to support your academic goals. The science & math classes offered for freshman year might be biology or earth science, geometry or algebra II…any of these courses would be helpful for you; just make sure that you have the correct prerequisites to enroll.
Good luck in high school! And remember to have fun!