To get started identifying your topic, the first thing you should do is decide which question you want to try and answer. For example, if you're interested in finding out about use of alcoholic beverages by college students, you might pose the question, "What effect does use of alcoholic beverages have on the health of college students?" It's important to choose a topic that you are interested in.
After you've identified your research question, look at what the most important concepts are. In the example above, those are "alcoholic beverages," "health," and "college students." These will be the keywords you'll rely on to search for datasets and journal articles.
If you get stumped trying to narrow your focus, try discussing your research topic with your instructor or visit one of your librarians. You can also look at the readings or assignments in your class to get an idea of what kinds of questions are being asked.