You can build your library of citations in a number of ways:
Note: Remember that Mendeley collects all references added to personal libraries to the Mendeley Web Catalog. If you don't want a reference added to the catalog, check the Unpublished Work box at the bottom of the details pane.
Mendeley Desktop automatically syncs to your Mendeley web account each time it's opened, but it's a good idea to sync frequently as you make changes to your library. You can change sync options by selecting All Documents in Mendeley Desktop. Then click Edit Settings above the middle column to specify which folders to sync. You can also deselect Synchronize Attached Files if you wish to preserve your Web Storage space.