Challenge yourself academically. Getting good grades in the classes you take is important; however, it is not the only part of your high school record that colleges will review. Which classes you choose to take is also important.
Read about time management. If you take on an intense workload, you'll need to know how to manage your life.
Take honors courses, but not more than you can handle. The object is to get good grades and learn. Taking honors courses tells colleges that you are serious about college and are willing to go that extra mile.
Enroll, if possible, in honors English all four years of high school. This is excellent preparation for all those papers you'll have to write in college.
Take three years of a foreign language. If your school does not offer a third year, or if for some reason you cannot take a third year due to a critical course conflict, you will need to explain this to your prospective colleges.
Read about the colleges that might be a good match for you. Countless college prep books - available at the library and at bookstores - offer detailed information about what particular colleges look for in student applications.