Whenever you use content or ideas that are not original to you, or that you've published before, you must properly acknowledge them.Your acknowledgement should adhere to a formally recognized style guide. Some of the most commonly used style guides are listed below.
For more information on how to avoid misusing information in this way, see this article from Duke University on plagiarism.
The Publication Manual of the American Psychological Association has been published since 1929 by the APA.
Purdue OWL Guide to APA
APA Style Tutorial
The Chicago Manual of Style (CMOS) is a style guide for American English published since 1906 by the University of Chicago Press.
Chicago-Style Citation Quick Guide
Purdue OWL Guide to CMOS
The MLA Style Manual and Guide to Scholarly Publishing has been published by the Modern Language Association of America since 1985.
Purdue OWL Guide to MLA
Kate L. Turabian's Manual for Writers of Research Papers, Theses, and Dissertations is a modified version of the CMOS.