Start with some of the key words and phrases of your research question.
For example, if your research question is: "How are U.S. states responding to the challenges of Climate Change?" you might start with "climate change" AND states AND policies.
Read through some of the articles you find, looking for additional ideas and key words that can be used to search again. Are there specific issues to pursue further? Different key words or concepts you might search?
Note: Some database records will also provide subject heading or descriptor terms to help narrow or broaden your search. Keep an eye out for these! They can also be extremely useful.