Confession: I often start a research quest with a simple Google search. Keeping in mind that not all the information I find on the web is going to be reliable, Google is still a good way to get my feet wet with a topic and gather the relevant vocabulary I need. Then I can move on to a more targeted database search.
Example
You stumble upon this article, and it peaks your interest. So you ask yourself:
Is there a relationship between gut bacteria and autism?
Step 1: Step 1: Break up this research question into two or three main concepts. Ignore extraneous words.
Step 2: Conduct an initial search with these words, and quickly scan the results.
Step 3: Using information from your initial search, add synonyms, acronyms, and variant spellings.
(gut bacteria OR gut flora OR intestinal microbiome) AND (autism OR autistic)
Try Subject Searching!
You are probably most used to keyword searching, in which you brainstorm search words and various synonyms.
Subject searching can be used in addition to or as an alternative to keyword searching. If you use the correct subject heading, you will capture synonyms automatically (e.g. "brain injuries" will find "brain injury" and "concussion"). How do you know what the correct subject heading is? You can look them up in many databases.
There are people whose job it is to read articles and assign subject headings to them (i.e. decide what they are "about"). It's kind of like assigning a hashtag to something and then being able to search by that hashtag (...kind of).