First, state your topic.
List all the words you know that are related to your topic.
List academic disciplines or professions that might be interested in your topic.
List aspects (or an aspect) of your topic on which you want to focus (discipline-specific aspects, location,
time period, etc.)
Narrow or revise your question if necessary. You may find that you will need to revise a few times as you go along.
Look over your work. Do any terms or keywords stand out as particularly meaningful? Group them into concepts. For instance, if your topic is: Do multiracial students have a harder time in school than single-race students? Your concepts are 1) multiracial, 2) students and 3) school environment. Add synonymns, related words, broader terms and narrower terms to your concepts. You can find related search terms using a thesaurus, browsing databases or even on Google. List keywords under the corresponding concepts. For example:
1) multiracial, biracial 2) students, children, schoolchildren and 3) schools, school environment, elementary, middle school, high school, elementary education
Choose the keywords you think are most relevant to your topic and use those to start your search.
Partially based on "Keyword Search Strategy" by Washington State University Libraries.