You can organize your references with Folders:
1) In your Mendeley Library, click on the Create Folder button on the menu bar at the top; or
2) Under My Library on the left-hand Menu panel, click on the Create Folder... link. This link is available in both Mendeley Desktop and Mendeley Web.
To add references to Folders, simply drag them from the main panel of the library and drop them in the Folder.
You can organize and rename the PDFs you have added to your Mendeley Library, specify a folder location to store your PDFs, set up Zotero integration, etc., with the Options window.
To start, in Mendeley Desktop, select Tools > Options.
1) General - Shows you how much storage you have left for your Personal Web Space.
2) Document Details - Specify the important fields related to each Document type.
3) File Organizer - Specify the folder location to store PDFs, as well as naming convention of PDF files.
4) Watched Folders - Select folders to watch so that new PDFs in these folders will be imported to Mendeley automatically
5) BibTex, Zotero, Connection - Configure BibTex, Zotero, and proxy connection.
This video from Mendeley shows you how to:
1) Sort your references in your Mendeley Desktop Library (0:15)
2) Read a PDF document in the PDF viewer , highlight text, and annotate (0:46)
3) Create a new folder (1:41)
4) Use filters (2:02)
5) Add tags and notes to references (2:43)
You can highlight and/or annotate a PDF which is in both Mendeley Desktop and Mendeley Web.
Double-click on the reference to open up the PDF you wish to highlight or annotate (add a sticky note). A new tab will be opened.
Select the Sticky Note icon to add a note, or the Highlighter Pen icon to highlight a portion of text. You can also select different colors for your Highlighter Pen.
Alternatively, select the portion of text which you would like to highlight. In the pop-up window, select the Highlighter Pen to highlight the text, or the Sticky Note icon to add an annotation.
You can view Sticky Notes annotations in the Notes tab (next to the Details Tab in the right-hand panel).
Do note that highlights and annotations are saved with the PDF in your Mendeley Library an is only visible in the Mendeley Library. To export the document with annotations, or to export the annotations, go to File > Export PDF with Annotations. In the pop-up window, select whether to export Article (with Annotation and highlighted text) or just the Notes. More details on this available at this Mendeley Blog Post.