I want to create a mod for (( lord of the ring the battel for middle-Earth2 : The rise of the Witch-King )) .

You take the code and you change it, to your preferences

How you go about getting access to the code I'm not entirely sure.

Most games that are moddable by the community come with a game editor.
Check here: Resources/FAQ [World Builder] - Battle for Middle Earth II Heaven Forum