You can create collections to organize your documents in whatever groupings make sense to you. Documents can be in more than one collection. By default, Mendeley has these folders:
In Mendeley Desktop use the folder icon on the toolbar to create new folders. In Mendeley Web click on +Create Folder in the left column.
Add tags to the record for each article to help you find articles within your library. You may filter your results by Tags as well as by Authors, Publications, and Keywords. The Filter is located in the lower left column.