Zotero is a free citation management software available for Windows, Mac, and Linux systems.
It enables users to collect, manage, and cite research from all types of sources. It is easy to use, and offers both a web version and desktop software version. Zotero allows you to attach PDFs, notes, images and screenshots to your citations, to organize them into collections for different projects, and to create bibliographies using Word, Google Docs, or OpenOffice.
To use Zotero, you can either log into Zotero online from their website, or download Zotero 5.0, which runs as a separate application but connects to your favorite browser after you install the appropriate plugin. The plugin, called Zotero Connector, works with Firefox, Chrome, and Safari. Zotero 5.0 includes the option to install plugins for Word and OpenOffice, so you can 'cite while you write'.
For more help, see this official Zotero guide on their website.