If you have not chosen a trend, spend some time searching Google and figuring out what interests you. For any trend you select, you'll want to make sure that there is at least some scholarly information on the trend, e.g. a research study conducted and reported on by an expert and most likely published in an academic journal. You can search Google Scholar early on to get a quick idea of whether or not the scholarly sources you'll need exist.
Wikipedia can be a great place to begin finding information on a teen trend or issue. You'll need to find additional sources, but Wikipedia articles may help you learn more about your chosen culture. Use the citation list at the bottom to lead you to more sources.
You may be allowed to use a few popular sources in addition to your top-tier scholarly sources. These newspaper databases will help you locate newspaper and magazine articles and possibly get access to back issues of publications that you wouldn't be able to get through Google.
You may also need to consider how to narrow your topic down, or how to relate it back to course themes such as adolescent development. For example, if you're interested in "teen use of social media," maybe you'll want to focus on a specific form of social media such as SnapChat. From there, perhaps you'd focus in how SnapChat, or other new media environments, affect teen relationships.