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" (green plus sign) button.
The item type you select when you create a record 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)
There are three tabs to the right of the "Info" tab that can be used to further organize your library.
Notes:
You can create one or more notes related to your record. Use these to keep track of Interlibrary Loan requests, books you have checked out, the locations where you are storing the book, storing citation information in non-Latin scripts, etc.
Tags:
Tagging can help you retrieve related records. For example, if you have multiple records that mention someone important to your research you can tie these together with tags. Once you have used a tag, it will show up as an option when you start typing. This allows you to reuse tags and improve retrieval. Searching on a tag will result in a list of every record in your library with that tag.
NOTE: Sometimes Zotero brings in subject terms from the databases and adds them as tags. Don't be surprised when you find that your record already contains tags!
Related:
The related tab is a great way to tie records from the same source together. For example, if you have a book and are using multiple chapters by different authors you can add the record for the book, plus records for each book section. In the book record, click on "Related" and add a link for each chapter. This will, in turn, create a link from the book section record back to the book. In your bibliography you can use the citation for the book, while in your footnotes you would want to cite the individual chapters. Add all the records first and then link them together. As you type in the related tab, you will find the record you need.