Keep in mind that research is an iterative process so you will have to do multiple searches in more than one database for maxim results.
Keywords are words that carry content and meaning. The keywords in the research question "What is the feeding range of the blue whale in the Pacific Ocean?" are feeding range, blue whale and Pacific Ocean.
Think of words similar to your keywords in case a database doesn't use your original keywords. Synonyms for blue whale are baleen whale and Balaenoptera musculus.
A Boolean search is a search using the words AND, OR and NOT between the keywords. These words have a special function when used in a database.
You can avoid doing multiple searches for variations on word endings using the truncation symbol * (the asterisk) in most databases. Entering the keyword "blue whale*" will look for both blue whale and blue whales.
If you want a literature review, add "AND review" to your keywords. To find a research study, add "AND study" to your keywords.
Always go to the Advanced Search in a database to enter your Boolean searches because it gives you multiple boxes with the Boolean operators between them. If you are using a search with multiple search strings, enter OR within the search boxes and AND between the search boxes, e.g., [blue whale OR Balaenoptera musculus] AND [feeding range OR feeding grounds] AND [Pacific Ocean].
When you don't know synonyms for your topic keywords, you can use online dictionaries, encyclopedias, and other reference books to find them. You can also use Wikipedia to find a definition or synonym for a word, but don't use it as a source for your paper or project because it is not authoritative.