You must realize that your high school grades and class selection are vital when applying to college. You should try to maintain a good grade point average through all four of your high school years. Your Junior and Senior year grades and class selection are extremely important. If you aim to apply to a competitive college, you need to maintain a high grade point average; anywhere from a 3.33 to a 4.0 would put you in the running. Of course, the higher your grades, the better chance you have to getting into your desired college. As far as class selection, you should take the most advanced classes that you can, as long as you can succeed in those classes. A "B+" in an honors level class may be better for many colleges than an A in a regular level course. Most colleges want to see that you challenged yourself during high school. Still, if you can't maintain at least a "B+" average in the honors class, the regular level class is probably a better choice. AP classes are also always beneficial when applying to colleges. Plus, you can take the AP test at the end of the school year and perhaps earn college credit for the AP class, which could save you money and get you out of some of the entry level college courses. Make sure that your junior and senior year you take a balanced schedule of subjects and you challenge yourself as much as possible.
Colleges also scruntinize your life outside of the classroom. Most colleges like to see that you were a leader in high school and that you took advantages of the opportunities available to you. Extracurricular activities such as sports, theatre, music and academic programs (such as National Honor Society, scholars bowl, student council etc), and community service are looked highly upon by college admission departments. You certainly don't have to be involved in everything. In fact, most admission departments would rather you really excel in one or two extracurricular areas than play a small part in many different activities.