Well, you shouldn't worry about that too much, depending on what the availability of classes is at your high school. You should have a strong root in science and math, both are critical to that sort of career. Some classes I would suggest taking that are offered at my school could range from all the following: Biology, Chemistry, Physics, Forensics, Environmental Science, Anatomy/Physiology, Algebra I, Geometry, Algebra II with Trigonometry, Pre-Calculus, Calculus, Law, Philosophy in Literature.
Of those I would say that Chemistry, Forensics, and Law would be things you would probably want to focus on. Generally, any core courses you take in high school will do, and college will help you learn whatever else you need to if you want to study to become a CSI.