Since the process of collecting information about different colleges, choosing the best options, applying, waiting to hear if your teenager is accepted, and applying for financial aid can take a lot of time, you should start gathering information in your teenager's junior year of high school and apply to anywhere between three and five schools in the senior year. You want to apply to more than one school, in case your teen isn't accepted to his or her top choice, or if your teen is accepted but the tuition at the school is too high for you to consider, even with financial aid.
Are you and/or your teenager interested in a local school or one that's affiliated with a particular religion? Does your teen already know what he wants to major in? In this case, you can look for schools that have strong programs in that particular field. Your teen may prefer to go to a college that her friends are going to or she may be ready for a change. Sometimes you hear the term "safety school" applied to a second-choice or third-choice school, as if the students think that the school is their safety net; the logic is that if their first-choice school doesn't accept them, certainly the next one will. This may or may not be true, depending on how many applications the college receives that year or whether they've changed their admissions requirements. The only real "safety" school is a community college, which is open to everyone.
You can go to college fairs with your teenager and speak to representatives from the colleges there, or the two of you can tour a campus (consider it some quality time with your teenager). Contact the colleges that your teenager expresses an interest in attending, find out what the tuition rates are and download the FAFSA (Free Application for Federal Student Aid) in January of the year that you'll want your teenager to apply for college.