Your professor will usually specify which citation style to use.
If your professor does not require a specific format and allows you to choose one, be consistent.
For example, if you choose to use the APA style, then you must apply that style to your entire paper.
Do not use different style formats within the same paper.
By citing, you are letting your readers, including your professors, know where you got your information, ideas, or thoughts from and, at the same time, you are crediting the author for the original information.
"A citation is the way you tell your readers that certain material in your work came from another source. It also gives your readers the information necessary to find that source again." -- From Plagiarism.org - What is Citation?
Basic Elements of a Citation
To use when studying the social sciences, behavioral sciences, and many other sciences.
MLA Style (Citation style from the Modern Language Association.) To use when studying literature and language and related fields in the Humanities.