From a database

In most online resources, Zotero automatically detects when you are looking at information for a book, article, or other source and inserts a "save reference" icon right into the address bar. The address bar icon will look like a book, document, newspaper, etc. To save, simply click the icon. If you are looking at a list of references, you may see a folder icon. This icon will prompt you to select multiple items from the list to save.

Add items manually

In some cases, such as a reference in print, you may need to manually add a citation to your Zotero library. To add a citation manually, simply click on the PLUS button and select the type of citation you would like to add. A blank template will be added and you can populate the fields with citation information.

Add items by identifier

If you have a DOI, PubMed ID (PMID), or ISBN number, click on the magic wand icon from the Zotero toolbar  and enter the number into the pop up window that appears at the top of your browser.

Add PDFs

Add a PDF to your library by opening Zotero, then drag and drop the PDF into the collection you would like to add it to. If the PDF has been created with Optical Character Recognition, Zotero can retrieve the metadata for the PDF. Either right-click (PC) or control-click (Mac) and select Retrieve Metadata for PDF from the drop down menu.

Full instructions: https://www.zotero.org/support/adding_items_to_zotero