Mendeley is a citation management tool provided by Elsevier. There are free and for fee versions of Mendeley.
First, get started with a free web account that offers limited features. If you do get a desktop, the web and desktop will synchronize.
The download options are described online. Be sure to select the Mendeley version that is right for your operating system:
Here are just a few guides from Mendeley and other academic libraries. They can help you get started:
Install Web Importer in your browser to capture citation information from PubMed, Scopus and other resources into your Mendeley web account.