Are you asking to buy DirectAdmin for Microsoft Windows? If so, it's not available; DirectAdmin is only available for FreeBSD and for Linux.
DirectAdmin runs on the main IP# of your server, normally in a datacenter, because it must be a static IP#. You find out the hostname of your server by giving the command hostname at the root shell.
If you're attempting to set up a DirectAdmin based server to do webhosting either for yourself and others and do not understand these basic questions then you should probably get a dedicated or virtual fully managed solution from a solutions provider rather than attempt to do it yourself; the learning curve can be quite steep.
We and others offer complete dedicated servers including full management, dedicated reseller accounts, and dedicated hosting accounts, and most of us offer shared hosting.
Jeff