Reference sources, such as dictionaries and encyclopedias, will be useful for refining your topic and coming up with keywords. Some of entries will contain bibliographies which you can use to locate key books and articles on your topic.
Before you jump into searching, it can be helpful to spend a few minutes thinking about your research question and brainstorming for keywords that you can use in your searching.
Example research question: How does use of social media affect the grades of undergraduate students?
With this example research question, the key concepts would be:
social media AND grades AND undergraduate students
For each of the keywords you recorded in your previous answer, think of synonyms or alternative terms.
Try to think of terms that are broader in meaning, as well as terms that narrower in meaning. These broader terms may help you expand your search if you are not finding enough information, while narrower terms may help you focus your research.
For this example research topic, you might identify these keywords:
social media → Facebook, Twitter
grades → academic achievement, GPA, academic performance
undergraduate students → college students
As you read information about your topic, note the suggested terms that come up in some of the databases. These terms might be good to use in future searches.