Start with a keyword search for a huge topic, for example, global warming, then read the abstracts (summaries) for the very first 5-10 articles you retrieve keeping an eye out for a more specific aspect of your huge topic. Revise your keyword search by adding and/or changing your keywords to reflect the specific aspect. For example, you type global warming into a database search box and find an article that mentions polar bears. You then do a new search global warming polar bears to narrow your search.
Start with a keyword search for a huge topic. Use the built-in database features (usually available to one side or the other of your list of results) to narrow your topic by date, additional subject areas, etc.
Please note that it is absolutely normal to revise your keyword searches several times when doing research!