Question: how to make a song in your music library your ringtone?
Answer: Connect your phone to your computer using a USB cable. ... 
In the window that pops up, find the "Ringtones" folder. ... 
Drag the music file (MP3) you'd like to use as a ringtone into the "Ringtones" folder.
On your phone, touch Settings > Sound & notification > Phone ringtone.