There are several methods you could use to get the information you need, including focus groups, mall intercept studies, and online research targeted to your local market (provided it is large enough). One of the primary considerations, however, is cost. You would need to have a minimal research budget of about $20,000 to perform several focus groups. One is never enough because a group is so easily swayed by the input of any one strong personality. Since the local mall is often a nexus of teenage activity, a professional research firm can assist you in setting up a study where teens are "intercepted" and asked specific questions. This also requires a healthy budget.
Another option is to hold your own events, invite teens you know, and set up opportunities to test product/service concepts. This has more in common with experiential marketing -- giving prospects the opportunity to interact with your product/service so they form a personal connection. Then set up a teen advisory group who are the first to get the inside scoop on new developments. They'll spread the word for you! And don't overlook the internet as a means of gathering info -- from your own website to social networking sites.