Add windows 2012/ 2008 Domain controller to existing 2003 domain

Microsoft often has step by step guidance for basic tasks on TechNet. For example, adding a 2012 DC:

http://technet.microsoft.com/en-us/library/jj574134.aspx

If there is a reason you are finding the documentation insufficient, it'll be easier for is to help if we knew where you were getting stuck.