Below you'll find the most frequently used research databases in Communication, but the resources you need depend on your topic. The library's databases are organized by subject, but ask me if you're not sure where to search.
Use subject headings to create narrowly tailored search queries.
Subject headings constitute a controlled vocabulary that can help you find sources by their primary subjects. The Library of Congress maintains this vocabulary making it standard across many different databases and catalogs.
For example, imagine you're developing a topic on theme parks. Searching that term in LC's subject heading thesaurus reveals the preferred subject heading, "amusement parks." Using the advanced search interface in a given database, you can change the search field from keyword to subject as pictured below. Searching "amusement parks" in the subject field returns all results tagged with that heading. Now cross-reference your subject search with keywords specific to your interest in amusement parks, such as the way they're marketed.
Caveat searcher! Anytime you construct a query like this you may filter relevant results. It's often best to begin with broad, keyword-based searches, then subject searching when you have a clear and well-defined sense of the resources you need. Research is a process of moving between broad and narrow levels of inquiry.