Zotero: The Tri-College Libraries recommend Zotero, a free online app that can format your bibliography, keep your citations organized, and even save your articles in the cloud so you can access them later from the library, home, or a cafe.
EndNote Web: The Tri-Colleges provide free access to EndNote Web.
Any librarian will be happy to give your a tour or a few pointers of the software.
The most commonly used publication styles are created by the Modern Languages Association, the American Psychological Association, and the University of Chicago.
No matter what style your professor requests, a citation manager can help you organize and cite your sources, and so can a librarian.