Transferring

Hello, I have two internal hard drives - one is practically full and the other is almost empty. I don't know what I can transfer over there (I have both itunes and windows media player with lots of music and would like to move both of those over) or how to do it. Would anyone out there be able to help me with that? Can I transfer whole programs over there or just files. Also, is there any way to merge the two so that once one fills up things automatically move to the second hard drive? Thanks!