When using the library's research databases, many (such as Academic Search Complete) provide the option to place the document's citation information into the the style format of your choice when you go to print, save or email it. Remember to select the APA format (or which ever style your professor requires) and you can save yourself some time. For online resources that don't have this option and when citing from print resources, print and online APA Style manuals are available.
Not sure how to cite a resource? Ask a librarian!
Publication Manual of the American Psychological Association by 
APA Style Simplified: Writing in Psychology, Education, Nursing, and Sociology by 
The Concise APA Handbook by