You need to know how to run and maintain (and not screw up) a Domain before
you create one. If you feel you can do that, then to create the Domain by
making the Server a DC using "Dcpromo.exe" from a command line.
I recommend *not* naming your new domain the same as whatever your company
might be known by publicly in "Internet-Land". Yes there are a lot of
arguments about that, but since this is my post, and I have "been there -
done that" enough times, that is what I recommend. Just change the three
letter suffix to something that isn't used on the public Internet,...like
use "*.loc" (loc for "Local") instead of "*.com" or "*.net". So to the
outside world you might be known by "mycompany.com" while in the private
side of your system you would use "mycompany.loc". This goes hand-in-hand
with the concept of using a Private Address Block on the private system
which cannot be directly access by a Public System without special NAT or
"proxying" configurations. It is a lot to try to communicate in an email, so
you'll either have to trust me or ignore me, it's up to you.
Choose the name wisely. Whatever you choose, you are stuck with,...it is not
easily changed afterwards. 2003 Domains can be renamed, but only if
Exchange has not been added to the Domain.