Zotero allows you to enter records two ways: manually (covered on this tab) and by downloading them from a database (see "Importing Records" tab).
To import a record manually, start with the "New Item" (paper and plus sign) button.
Don't see your source type listed? Pick something that is similar.
The item type you select will determine which fields are available. You do not need to have information in every field, but you should at least have the basic citation information for that type of source.
Example 1: Book (basic fields)
Example 2: Article (basic fields)