To get started with Mendeley you first need to create an account
Then you have to
How to make an account and install Mendeley Desktop
From Mendeley Desktop - Tools install the Web Importer
This will take you to Mendeley.com/import where you can download the Importer and add it to your bookmarks toolbar
Then install Cite-o-Matic, which you find under Tools - MS Word Plugin.
If you are using OpenOffice, it will be possible to download a Cite-o-Matic for OpenOffice
In Word, you'll find Cite-o-Matic under the tab References