"File management" (which is what moving files is), is a lot harder to explain than it is to do.
Basically you open a window showing the folders on your C drive, then open another window showing the folders on your D drive.
Then you basically COPY the files from one window to the other.
It is best to COPY them first (like Vagrant says) just in case the first move does not work.
Once you KNOW they have been copied succesfully to D then delete them from C.
If you are not to sure about this try copying one file at a time first till you get the hang of it, then as you get more confident copy many files, then eventually whole folders.