Encyclopedias and other reference sources are a great way to quickly learn about your topic. They will help you develop more keywords to search, as well as pointing you to other resources in their bibliographies.
Use brainstorming to come up with key words that will help you expand your search. Try to think of related topics, both more general and more specific, as well as synonyms for all of your terms. For example fashion could become: dress, clothing, costume, haute couture, la mode, Chanel, Christian Dior, etc. A mind map can often be a good way to start thinking of terms and topics:
Online tools for creating mind maps: