Look for the database's Help page.  There is usually a link to the Help section of each database (e.g., Look for: Help, Tips, FAQ, Advanced Search). They usually provide good examples of how to search.

You can always ask a librarian as well.

Before you begin using a library database or search engine, it is a good idea to write down all the keywords and phrases that describe your topic or the information you are seeking. You should also write down any synonyms or related terms. These keywords and phrases can be your search terms.

Example: Does television viewing encourage aggressive behavior in children?

A simple search can be: television AND "aggressive behavior" AND children
A complex search can be: (aggressive OR aggression OR fighting) AND (children OR adolescents)

Other schools have put together demos and tutorials on various search techniques.