To select keywords for a search, start by writing your topic out as a sentence or question. For example:
"What role do supernatural figures or forces play in quest narratives?"
There are several key concepts in this question. First, extract those concepts from the topic question (bold). Then list synonymous or related terms for each - you have a list of keywords to use!
Getting started on your research? This video will show you how to come up with keywords based on your research topic.