THINK first, SEARCH later.
Always remember: KEYWORDS are critical!
Here's an example.
My topic is "Females have brought a positive element to the military."
My keywords in this topic are "female" and "military" and maybe something like "benefit".
My "related words" lists might look like this:
So I could mix and match - pick one word from each list and I could get great results from each search!
OR - if I wanted to be even more specific, I could make a phrase: <"Female soldier"> and then add a word from my third list: <benefit>. My full search would look like this: <"female soldier" benefit>
See how that works?
One thing to keep in mind when you are doing research on your topic is that there is NO SUCH THING as a single perfect source that you will be able to cite to support your thesis. Researching and writing is a process of learning about your topic, thinking critically about what you've learned, and supporting your thesis through referencing a variety of sources.