|I'm not sure what "thunderbird sharing" is but the first question is, does it actually require a domain?|
If it does, then you need a server running a server operating system on it like Windows 2000/2003/2008. To create the domain, you run dcpromo on the server.
However, before you begin, there's a lot you should learn about domain management, security, account management etc. So I highly recommend you do the research and reading before you attempt to begin. That or hire a professional consultant who already knows all of that and pay them to do it for you.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.