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 café.
For details about citation styles for different fields in the sciences, see the 5 drop down pages under the Sciences tab. You will find manuals and tips for the official styles used by the different disciplines.