Citation is your indication that certain material in your work came from another source. It also provides readers with the information necessary to find that source again. Giving credit to the original author by citing sources is the only way to use other people's work without plagiarizing.
The following situations almost always require citation:
Adapted from: Plagiarism.org
A citation serves two main purposes: it gives credit to the author, artist, or creator, and it allows your reader to find the book, article, or photograph.
A style guide shows you how to format your footnotes, bibliographies, or works cited lists. The style format you use (ALA, MLA, Chicago) depends on your field of inquiry.
Citation style manuals are available at the Reference Desk.
Publication Manual of the American Psychological Association by 
A Manual for Writers of Research Papers, Theses, and Dissertations by Citation Managers can help you keep track of your references and share your citations. They can help you create a bibliography and organize your work more easily.
