Take a course in French. Your high school, for example, might offer French as a foreign language. You can also look for courses at community colleges and universities once you graduate or at community centers if you are still in high school. Private tutoring is another option. Having a French instructor who knows the language well will help you learn the rules of the language.
Get a study partner. Learning a foreign language with a partner will go a long way in improving your language skills. Make a point of speaking French with your study partner as often as possible, ideally on a daily basis.
Read French literature. When you are just starting out, you can read simple grammar books or children's books. As you develop your skills, you can move on to adult literature, such as news periodicals or novels.
Look up French words in an English/French dictionary when you don't recognize them. Looking up the word will help ingrain it in your mind.
Take advantage of programs designed for teens that help you learn the language by traveling to France. Immersing yourself in French culture is one of the best ways to learn the language. Such programs are offered by organizations like Atoll Juniors and Souffle Juniors. Once you are a freshman in college, you might also be able to take advantage of a study abroad program.