There are various ways to add references to Zotero.
By Icon: If you have installed either Zotero for Firefox or Zotero Standalone along with a browser extension, an icon (a different icon for each type of source) appears in the browser toolbar. All you have to do is click on it and all information is imported.into your Zotero library.
By Identifier: ISBN (International Standard Book Number) for books, DOI (Digital Object Identifier) for digital documents. DOIs provide stable, persistent links to documents. They can be found on a digital version of an article and often in online databases. PMID (PubMed Identifiers), which are unique identifiers assigned by PubMed to the articles and abstracts in the PubMed database
Manually: You manually enter bibligraphic information
Books, Articles and More
Zotero senses when you are looking at an article in a subscription database or a newspaper website, a video on YouTube or Vimeo, or a book in the library catalog or at Google Books. In fact anywhere you go on the web, Zotero will be able to extract metadata from the site which allows you to add it to your library with just one click.
To save references to your library:
Look in the tool bar for the icon representing the webpage you are viewing. 
The icon image will change to a book, newspaper 
,magazine
, journal
, or a simple web page
(or others) depending on the type of resource you are viewing online.
Simply click on the icon to add it to your Zotero library (please note, Zotero must be running simultaneously with your browser).
In some cases, particularly in library databases and catalogs, a page with search results will prompt Zotero to show a folder icon
. Click on the folder to choose the search result item(s)that you would like to save.
