Ideally, you'll use the more efficient method in the next section, for adding citations into your library directly from databases. But sometimes you may need to add them manually. This video explains the process of adding citations manually.
The Zotero Connector that you installed after you downloaded/installed the Zotero client is one of the super-helpful features. It allows you to download citations (and often the PDFs) directly from library databases into your Zotero library. When you're in a library database such as WorldCat Discovery, Academic Search Complete, JSTOR, and almost any others), looking at a specific citation, you'll see an icon at the top right of the browser. The icons indicate the citation type: