If Zotero detects that you're looking at a book or article on a library catalog or database, or a site like Amazon.com or the New York Times, you'll see a book or page icon appear in the address bar of your browser. Just click the icon and Zotero will automatically save the citation.
If you're on a page of search results with many items, you'll see a folder icon instead. Click this to get a list of all the items on the page, and check off the ones you want to save.
Zotero automatically saves PDFs along with citations when possible (you can change this in the preferences menu if you wish.) Attachments take up space in your Zotero account -- you get 300 MB free and can buy more space if needed.
You can attach files manually by dragging a PDF (or other file) into your Zotero pane. Dropping a file onto a collection, or in between library items, will copy it into your library as a standalone item. Dropping it onto an existing item will attach it to that item. This is the easiest way to attach a copy of an article to its entry in your library.
You can also add attachments to citations by right-clicking on the citation and choosing "Add Attachment."
Other Web Sites
Zotero can't automatically capture citation info from most web pages, but you can still add them to your Zotero library.
Click on the Save icon. Very basic information, such as the URL and date accessed, will be added to your library.
You can edit the fields to include more details, such as author (if there is one).
By default, Zotero will attach a "snapshot" of the page to the citation. Taking a snapshot saves a copy of the page to your computer. It includes the page's text and images, so if the page is removed later, or if you're offline, you'll still be able to view your copy.
Click the Zotero button at the bottom of your browser to open your library. At the top left is a folder button with a green plus sign. Click this to create a new "collection."
Create collections to organize your references. Collections are like file folders on your computer, but a reference can be in more than one collection at a time. So, a book on circuit design could be filed in your "Electrical Engineering" collection, your "Circuits" collection and your "IPRO" collection without having to add it three times and so that any changes made to the citation will show up no matter what collection it appears in.
There may be times when you need to add an item to your library by hand. If so, use the green "plus" icon and choose the type of item from the dropdown list. A window will open with the appropriate fields to complete for that item type.