If you already have some PDFs saved from past research, just drag and drop them from a desktop folder into your Zotero library. Zotero will automatically search online and match the PDFs with citations if possible. Right clicking or command+click will also produce a menu where you can attempt to retrieve metadata for specific PDFs.
Note: It's likely you will need to edit some entries manually. And, if Zotero cannot harvest metadata from the web, you will need to manually add those citation details.