Use reference books (encyclopedias, handbooks, dictionaries etc.) to get an overview of topics you may be interested in researching. The more background information you have on a topic of interest, the more you will be able to think of promising research questions. The information in reference books tends to be factual or widely accepted information, rather than critical and or debatable information and ideas.
Gale Virtual Reference Library
Lithograph poster, [c. 1895]. Library of Congress Prints and Photographs Division, Washington, D.C.