Zotero is a free, open-source tool you can use to keep track of the sources you find, organize them, and create citations.
To get started, you'll need to download both Zotero and the Zotero Browser Connector. Both downloads are found on this webpage:
The Virginia Tech tutorial below will help people who want to be power users of Zotero. It comprehensively covers almost everything you'd want to know about using Zotero.