The "Publication Manual of the American Psychological Association" is the primary resource for how to cite in APA style. You can find detailed guidelines in this manual for citing any source you might use, for example, single and multiple authors, or online or paper sources. The APA periodically updates this manual to reflect changes in technology that impact writing and publishing. The sixth edition was published in 2009 and was the most recent edition as of June 2011. Errors in the first printing of that edition have been corrected in subsequent printings and are available on the APA website.
APA has a website (apastyle.org) that provides concise but limited information on APA style. This website has two free tutorials on APA Style. One tutorial provides basic information for people unfamiliar with APA style. The second tutorial is for people familiar with APA style and demonstrates the differences between the fifth and sixth editions of the manual. You can sign up through the website for an online APA style course, which costs $40.
APA provides a number of products to help students and new users with APA style. For example, the APA publishes a self-paced workbook, "Mastering APA Style: Student Workbook and Training Guide,;" an instructor's manual, "Mastering APA Style: Instructor's Resource Guide;" and a pocket guide, "Concise Rules of APA Style." All these products are available for sale from the APA website.
Many colleges and universities provide writers with free online help with APA-style citations. Some of these sites give specific directions that are simpler and more concise than the "Publication Manual of the American Psychological Association." The Purdue Online Writing Lab (owl.english.purdue.edu) is particularly detailed and informative. Another online source, the Vanguard University Department of Psychology (psychology.vanguard.edu), provides a Microsoft Word template for a paper in APA format. Many research methods textbooks include sections with specific but limited information on APA citation style.