You can attach notes either to collections or items in your Zotero library. Select an item or a collection in your library, then click on the "New Note" button above the center column. Choose "New Standalone Note" to add note to collection folder, or "Add Child Note" to add to selected item. You can also click on the "Notes" tab in the right hand column to add a note to a selected item. Add your note in the right hand column.