Do background reading to learn more about your topic.
Start with a specialized Library Research Guide. These include lists of specialized research tools, what databases might be useful, as well as web sites and other resources.
Use specialized encyclopedias or dictionaries or reference books because they contain valuable background information that might give you ideas on how to narrow a topic, as well as bibliographies that can direct you to other sources on your topic.
Search for review articles on your topic. It will synthesize current research on a topic and lead you to many other sources.