Zotero (pronounced "zoh-TAIR-oh") is an application that collects, manages, and cites research sources. It's easy to use, connects with your web browser to download sources, and best of all it's free.
Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects and create bibliographies.
This is a very complete guide that covers most Zotero possibilities. You may not need all of it. Start with this first page and use others if you have questions.
Just the basics: this video will show you how to get started.
For more complete information: Watch this online workshop