How to change ringtones for music

Hi all,

This is my first post and I have a question concerning my Cell phone a Samsung Galaxy Ace 2 GT-I8160:

I downloaded a song from my PC to my sdcard0 under music storage. The problem is I can't find a way to transfer it to settings-sounds-ringtones. There is a list 100 items but when I press to select my music, an empty window opens and I don't know how to place my song between the ringtones list.

Any help would be appreciated!

Thanks in advance.