
Zotero is a free app that helps you keep track of the sources you've found, and lets you generate citations automatically. It's available for Windows, Mac, and Linux.
If you don't want to download an app, go to zbib.org to use ZoteroBib, a simplified web version of Zotero.
Caution: If you use an app or website to generate citations automatically, be sure to proofread the citations before you submit them to your professor. Automatically-generated citations can contain errors.