The APA Guide can be found in the library catalog under the title Publication Manual of the American Psychological Association. The Sixth Edition is the current one. If you're a psychology major, it's worth it to buy a copy to keep for reference.
One of the best online guides is the Owl at Purdue APA Formatting & Style Guide. If you need a quick template, or are having trouble navigating the Publication Manual, the Owl should be able to help.
Most citations for peer reviewed articles tend to follow the format for periodicals with a DOI. If there is no DOI, APA prefers there to be a URL from which you retrieved the article. However, instructions from your professor should always take precedence. Be sure to ask whenever there is a question.