Think of a topic or question that you're really curious about, that really compels you.
Write down some keywords and phrases that come to mind. You can use these to search. Be open to the possibility of new keywords, and try out different combinations!
Explore backgrounds and contexts. Reference sources can help you with concepts and terms, and they can also lead you to other relevant sources.
Search for books and articles on your topic. Carefully skim each source---including the synopsis, abstract, table of contents, and/or index--- to assess how relevant and useful it will be to you. Put back books or articles that aren't useful.
Document and cite your sources as you go along - this will make the paper-writing process much easier!