You can use anyone of several Apps to create ringtones from music. You have to select a segment of the song, since ringtones are limited to a certain length. In these apps you create a ringtone from a song on your phone and then follow instructions on how to sync that ringtone back to your phone using iTunes. You cannot simply point to a song on the iPhone to be used as a ringtone.
i wonder is there some way to set my own songs or music video as ringtone for my iphone?