These databases will generally start you out with a basic one-field search not unlike what you see when you go to Google. Don't fall for it. There are 2 big benefits to database searching: you don't have to scrutinize your results for reliability, and you have the power of the Advanced Search to weed out the irrelevant.
When you're constructing your search, think about how to break your topic down into little bite-sized chunks. You can't just write "Civil War Sherman burns Atlanta" -- well, you could... but it's more effective to break it down, as shown below:
As you add on additional fields for search terms, they'll be joined together by AND by default. This "AND" is the most powerful limiter: the articles you get back have to have term 1 AND term 2 AND term 3 to end up in your results.
If you notice you're getting a lot of articles about Gettysburg instead of Atlanta, you can add another field and switch the AND to a NOT (NOT Gettysburg) to help get rid of those results.
All those terms and phrases and ideas you developed before? They all can feed the Advanced Search machine. Be sure to consider synonyms for your different search terms so you can swap them out to play with your search. (E.g. "college" but also "university" or "higher education")
Plus, because you did that background research, you're starting off with a clearer idea of what information you need to find. This means you'll know what you need to put in those search fields to narrow it down from the massive and generic "Civil War" to "Civil War AND United States AND Sherman AND Atlanta."
You can also power-search Google using its Advanced Search page (or by learning a few of the special search commands).
One of the most impactful commands to weed out websites is the domain limiter. By specifying you only want Google to search fbi.gov or .edu, you'll boost your odds of getting reliable results.
Compare the following searches:
Other useful tricks include placing parts of search inside quotation marks to ensure it gets searched as a phrase, e.g. "white collar" crime. If you want to emphasize a term you really want to include, place a plus sign in front of it (+crime) or a minus sign to exclude a word (-arrests).