You can use a song as your phone ringtone, in place of one of the ringtones that come with the phone.
Touch and hold a song in the Music library list. In the context menu that opens, tap Set as ringtone, then tap Phone ringtone or Caller ringtones.
OR
On the Music player screen, tap the Menu icon , tap Set as ringtone, then tap Phone ringtone or Caller ringtones.
If you choose Caller ringtones, tap the contact entry to set it.