There are a variety of ways that you can add items to your Zotero Library. It depends entirely on where you're starting from and how you like to work.
We've outlined how to use the Browser Connector below since it is the easiest and most common way of using adding items to Zotero.
But there are several other options for adding items to your Zotero Library that also might be useful:
This documentation from Zotero covers every way to add an item from the basics outlined here to doing large scale imports.