Try this How to Generate Keywords tutorial, from the University of Texas at Austin. It will take you through the steps of brainstorming synonyms for the topic of your choice. You can experiment with searching for these terms using the ISU Library website.
Sometimes the topic you’re researching is so new to you that you don’t know whether you’re using the right keywords. When that’s the case, finding background information can help. Background information can lead you to new concepts and keywords related to your topic. Believe it or not, Wikipedia can be a good source for this type of information.