This screencast on this page details all of the ways to import bibliographic information into Zotero, as well as how to enter information manually if none of the other options are available. Briefly, it covers
importing from library websites, databases, etc. using Location Bar Icons;
saving data and a snapshot of a web page using the Create New Item from Current Page tool;
retrieving metadata from a PDF on your computer;
looking up items by DOI, ISBN, or PubMed ID;
adding information manually; and
importing records from another reference management software tool (e.g., EndNote or RefWorks).
"Getting Stuff into Your Library" presented by Zotero