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. The current edition for APA is the 7th.
This is a great resource for examples of how to cite different kinds of resources, to get an understanding of the differences between in-text citation and the reference list, and also better understand what plagiarism is and how to prevent it when using APA style. The APA manual is more in-depth with explanation, but this is a good reference for many common citation questions.
Watch the Tutorials and Webinars on APA Style.
The APA Style website also has their APA Style Blog, which includes blog posts answering questions that come up for APA citation that are commonly asked.
Below is a reminder of some common formatting details that sometimes are overlooked.