It's never too early to start thinking about applying to college. The early, the better. You don't want to be a senior and regret not doing this or that. Regret is the worst feeling. You can definitely avoid regret by careful planning and preparation. Below are a few things you can start planning or doing early on in your high school career that will be useful in applying to college:
Students are scared of the SAT only because they usually aren't prepared to take the test. If you start studying for the test at an early age, getting a good score is highly attainable. It is not like the IQ test which doesn't require studying. There are courses (Princeton Review, Kaplan), practice tests, books that can increase your score tremendously.
Don't waste your summers. Your summers can be a great time to learn something new, volunteer, develop leadership skills, participate in an essay worthy activity, make some money...etc. What you did in the summer can speak volumes in an application.
One of the best ways to get into the college of your dreams is to research several colleges and know why you selected your top choices.
Pick a few extracurricular activities that you are excited about. Colleges like to see consistency on a student's application. So when choosing these activities, make sure you are choosing them because you are passionate about them. You will eventually want to hold a leadership role by the time you become an upperclassmen.
"Reading is good for the soul". Since you don't have homework, take some time each day to read something. Reading is not just a good way to learn new things but it also keeps the mind fresh. Reading also helps you with your imagination and beings to help you develop your opinions about certain issues. Try a mix of nonfiction, fiction, blogs, newspaper, and trade magazines.