Hello again
Okay, lets start from scratch.
A FQDN is not a configurable option in the sense that a computer name is. It is a term that defines an objects location in a DNS namespace.
Therefore, you decide on the name that you are going to assign to your domain, i.e. companya.com ( I know that you do not HAVE TO do this), but if you have a web presence, or intend to have one, register a web address and use it as your domain name. It is better to be pre-prepared!!
So, you now have a Domain name. The next step is to build the First server in the Domain, as stated by ScreeminChikin. During the DCPROMO process, you will be asked to install the DNS service.
Once DCPROMO is completed, you must then Configure DNS. It is best to setup as an Active Directory Integrated DNS Zone. Once you have then created the Forward Lookup Zone for your domain namespace, you will have an FQDN for all named objects on your Domain. i.e. server1.companya.com
To be honest, if this is information that makes little or no sense, I would look to get Tech help from your nearest Microsoft Certified Partner, or get on a steep learning curve at the MS Active Directory Website.
Adrian Jordan
MCSE,CCEA,CCNA