The Goldilocks approach: Not too broad, not too narrow, but juuust right.
The 5 W's approach: Narrow your topic by thinking about Who, What, When, Where, and Why.
The Advanced Search approach: Searches with one term answer "what is...?" type questions. Searches with at least two terms answer more complex questions that you can take a position on.
The 1-minute approach: You have a good working knowledge of your topic when you can talk for an entire minute without stopping.
Depending on your topic and your knowledge about it, you may want to find an overview. An overview can help you narrow your topic, put it in context within its discipline and identify "classic" research. Use the sources below to start or ask a librarian for help.