Once you run the dcpromo or the configure your server wizard for AD,then go to the other computers to add to the domain and right click my computer and properties.Select computer name tab and hit the change button,use the admin account and password from the domain to join it.
In AD there is already an Admin account built in use that name and pass or create an account and add the admin group to the account and remove the users group from it.Then use that account to join the domain.