Hi,
You will need a laptop to do this. if the music is on google play music then go to play.google.com. Log into your account and select your song. Click on download. Then, open the file. Connect your phone to your laptop and allow your phone to receive files. Access your phone's storage and drag the song into the "music folder".N.B. make sure the music is in mp3 form. Allow this to sync and then disconnect your phone. Download a ringtone making app, there are many on the play store. Now,find the song with the app and set it as your ringtone.
I hope this helps.