Do you have a physical copy of a book or a journal article that you want to add to your Zotero library collection? You can do so manually, by clicking the New Item button (
) in your Zotero Library, selecting an item type and filling in the available fields. You can also have Zotero do this for you, by clicking on Add Item(s) by Identifier (
), and filling in the ISBN or DOI of your resource. Zotero will then populate the rest of the fields.
As well, you can manually add notes to any item in your Library by clicking on an item and then selecting the Notes tab in the right side panel. This will allow you to fill in additional information, personal notes, or things you wish to communicate to those with shared access to your Zotero Library.
Check out Zotero's Support page for more tips and tutorials.
York University Library's Zotero guide has additional information that is not discipline-specific.