When you have a thesis or research question in mind, start by identifying the most important words or phrases in your topic.
Then, brainstorm additional keywords that are synonyms or closely related terms to each of the concept words or phrases you identified. Later, you will be able to try different combinations of these terms in your search. Different articles may use different terms, so brainstorming a variety of words will make sure you don't miss useful articles.
You can use this worksheet to brainstorm terms for your topic.
Interactive Virtual Brainstorm Worksheet
Printable Version of Brainstorm Worksheet
In addition to reference sources, Credo Reference has a mind map feature that may help you identify synonyms or related terms. For science topics, you may end up using both common and scientific names in your searches.