In Windows Server 2012 go to the Start Menu and select Active Directory Users and Computers
Select the Organisational Unit (Department) where the user is physically going to work. This is important because business rules such as a person's default printer and what icons they should have on their desktop will be delivered to this user via Group Policy based on the Organisational Unit this person has been set up in.
Now click on the "new user" icon at the top of the window.
Enter in the Users details and press Next
Define the Users password. I suggest creating an initial password and then asking them to change it.
Press Finish when ready.