This section from the APA website provides a quick resource for citing references in papers using the 6th edition of the Publication Manual of the American Psychological Association (2009).
APA Website - Quick answers for formatting and references
APA citation style refers to the rules and conventions established by the American Psychological Association for documenting sources used in a research paper. The APA Citation Style is often used for courses in psychology, education, business and other social sciences.
DOI numbers are now required for APA citation if they are available. For more information on how to find a DOI number, follow link below for a tutorial from the APA Style Blog:
Writers Handbook - APA documentation style from the University of Wisconsin-Madison Writing Center.
Online Writing Lab - from Purdue University. Lots of advice for writing, tips and examples on paper setup, as well as APA citations.
Guidelines for Writing in APA Style - From Southern Mississippi University. Lots of examples for print resources. Good outline for formatting papers and general guidance as well.