The American Psychological Association (APA) has established a style that it uses in all of the books and journals that it publishes. Many others working in the social and behavioral sciences have adopted this style as their standard as well.
APA's style rules and guidelines are set out in their publication manual:
Citation styles change over time. Be sure to only use the most recent resources that reflect the current edition.
Watch the Basics of APA Style tutorial.
APA Formatting and Style Guide
From OWL - the Purdue (University) Online Writing Lab.
Here is a sample paper provided by APA.
For every style rule, there is a comment highlighted in the paper that tells you where to find the discussion of the rule in the APA Style Manual.
Below is a reminder of some common formatting details that sometimes are overlooked.