Your zotero library is divided into 3 sections:
"My Library" holds a copy of all the references, which you have imported to your collections.
To keep track of your references use "New Collection" to make folders in your Zotero library.
To import metadata such as author, title etc. to a specific folder, mark the collection ahead of importing.
To make sub-collections, right-click on a collection and choose "New Subcollection":
