Provided your server is already promoted as a Domain Controller, you need to open the Active Directory Users and Computers console. This is located, on the server, at Start > Control Panel > Administrative Tools > Active Directory Users and Computers.
This will open the console, where you need to double-click on your domain name in the left tree to expand it. From here, you can either right-click on ANY container (including the root domain), choose the 'New' option from the Menu and then choose Organization Unit. At this point, it is similar to making a new folder in Windows Explorer.
-tigermatt
