For tunes pics, videos and other data simply drag and drop or Cut and Paste.
Program files wont migrate like this. They are integrated with Windows registry on C:
If you move all the data over to nice new folders on D: you will probably have cleared a large proportion of the possible space.
If you want to move applications you must uninstall them one at a time and re-install them to Drive D: You do have a choice but most times the default C: is OK-d.