You can set a song from the
Music app's library as your regular ringtone or as ringtone for a certain contact.
From the Home screen, tap
Browse for the song that you want to set as a ringtone and then play it back.
Now playing screen, press and then tap
Set as ringtone (or
Set as ringtone options menu, tap
Phone ringtone or
If you select
Contact ringtone, you need to choose the contacts you want to associate the song with in the next screen.