Narrow it down to a specific time period if necessary. Articulate the possible approaches and select the best one for your needs. Determine the disciplines involved . Most topics are interdisciplinary. The disciplinary focus will determine the appropriate tools to use to locate information.
Encyclopedias cover different aspects of a subject.
Subject encyclopedias for your discipline are a good place to start. Try to look at topics like these from a new perspective or angle. Once you have your topic, write it out as a preliminary question or statement of purpose . Formulate the questions you want to ask. Remembe that there is not just one correct thesis.