Depending on your topic, you will want to explore one (or more) of these Library Subject Guides for discipline specific resources.
Online databases and other library resources often utilized Standardized Subject Headings to help organize information and help researchers easily find all the information on a relevant topic.
But, you do not have to know what subject terms use to before you start your search. One recommended strategy is to start with a keyword search, and then browse the subject terms represented in your results to see what additional search terms you should use.
For an example, keyword search for "older adults" AND "exercise" in the database Academic Search Complete leads to over 11,000 articles. Below is a screenshot of the subject headings attached to an article.
These subject headings can verify that I am using the right search terms, and can help identify any alternate search terms I may want to use. You can use the "Subject: Thesaurus Terms" Filter on the left side of the screen narrow your search to only those articles with particular subject terms.
The database PubMed has its own subject headings called MeSH Terms, or Medical Subject Heading Terms. They terms are slightly different than what you may find in Academic Search Complete, so the terms that were helpful there might not be as helpful in PubMed. You may want to start with a keyword search here as well, to help ID relevant MeSH terms to your topic.