Always use the citation style that your professor requires. If you are free to choose a citation style, use one you feel comfortable with, and use it consistently and correctly for the entire assignment.
Many of the resources on this page are style manuals - they provide information on all aspects of formatting papers in a particular style. These manuals usually have 1 or 2 chapters on how to cite sources.
APA is used in the social sciences. It is also commonly used in business and education.
Publication Manual of the American Psychological Association by AMA is used in medical and health fields (with the exception of pyschology).
AMA Manual of Style (Online)MLA is used in the humanities.
MLA Handbook by ACS Style is used in Chemistry. This guide is only available as an online resource (linked below).
Chicago is used in the humanities, especially History. (If you have been told to use Turabian style for citations, it is almost identical to Chicago.)
The Chicago Manual of Style, 17th Edition by IEEE is used in Engineering.