APA Style is typically preferred for psychology, but check with your professor for style requirements and recommendations to be sure.
Plagiarism is the act of using another person's words or ideas without giving him/her proper credit. It is a serious academic offense.
The Academic Integrity page defines plagiarism in more detail and provides information about Muhlenberg's Academic Integrity Code.
Indiana University's plagiarism tutorial also offers helpful information on understanding and recognizing plagiarism, including an overview with basics and recommendations and examples.