After you have identified your topic, it is important that you make a list synonyms/alternative terms so you can more easily find books, articles, and more. Remember this will help in Google too!
For instance, while you may say teenager, some sources may use adolescent instead.
Did you know you can use Wikipedia to do this? Watch this short video. (Just remember not to cite Wikipedia in your speeches!)
Having trouble thinking of a topic that interests you? These resources will help.