It's easy to get started. Just go to www.mendeley.com, fill in the requested information, and then click on: 
Mendeley has two platforms. Some tasks you can do in both, but each has a particular focus.
Mendeley Desktop is where you can organize your references in more detail and annotate the PDFs you collect.
Mendeley Web is your sharing portal. It enables you to easily access references and share your research with others.
Mendeley for iOS allows you to read and annotate PDFs and sync those annotations with your Mendeley Desktop.
To take full advantage of Mendeley features you will be using both.
A Sync button in the tool bar of Mendeley Desktop allows you to update your work to Mendeley Web.
On the web interface, citations offer a link to the desktop interface.