Use keywords and synonyms:
Start by taking keywords directly from your thesis statement.
Think of synonyms and related concepts for the keywords in your question.
Ex. college OR university
Ex. global warming OR climate change
Ex. marijuana OR cannabis
Look at the subject terms in your search results and see if you can use any of those.
Use truncation:
You can broaden your search results by typing an asterisk symbol * at the end of the root of a word. When you do this, the database will search for alternative endings for the word you have typed.
For example, perm* yields results for:
permit permitted permissible
Another example:
legal* legalize(d) legality legalization
Narrow your topic:
Broad topic: Mental illness
Narrowed topic: personality disorders
Research question: Is borderline personality disorder more prevalent in women than men?
Each of these sources has its own strengths and weaknesses.