Mendeley is a free desktop and cloud application that helps you manage your research. It is a reference manager with built in tools to help you read and annotate articles. It allows you to organize PDFs, other documents, and citations into folders and collaborate with other researchers. It backs up your research library and syncs to the web, and smart phone or tablet devices. It is also a social network for scholars across the globe with an online research library that you can use to discover research papers.
After downloading and installing Mendeley Desktop, I recommend you install the MS Word Plugin and the Web Importer. Instructions for installing the plugin are on the Importing References and Web Importer pages of this guide as well as on Mendeley.com.
There are lots of ways to get help from Mendeley. In fact, I've even tweeted to them and gotten assistance! The Support Forum is probably the best place to start. Search for your question in the search bar or browse the recent and frequent help topics.