Gather all the data. Scour the U.S. News and World Report's annual college-ratings issue, find reference books about colleges in your school library and find as many websites as you can that give voluminous amounts of data about colleges. Know which colleges excel in the field you think you want to study.
Realize your strengths and limitations. Know not only your SAT score, ACT score and GPA, but also your top college choices' average and low-end GPAs that were accepted in the previous year.
Ask your school's guidance counselor for help. Whether or not you actually take his advice, he can lend some practical help. For example, he can arrange tours for you, get you in touch with the right people in admissions and give you objective feedback about your decisions.
Figure out your comfort level with regards to geographical location. Some students want to remain as close to home as possible (even living with their parents throughout college), while others want to go as far away from home as possible. Only you can make this deeply personal choice.
Send out several applications. You might not get into your top-choice college, so have some backups in mind.
Visit the colleges that make your "top list," if feasible. Often the act of visiting a college and seeing it live and in person can make a world of a difference when selecting a college.
Look into the cost of the colleges. College is expensive, and you or your family will have to pay for it. Apply to as many scholarships as you can and file the Free Application for Federal Student Aid (FAFSA) as early as possible. There are many resources for making college less expensive.