The best thing to do would be to check out your phone's user manual.
If the song is on the phone already (you have to download the mp3 music file for the song), you should be able to set the ring tone.
Play the song from your phone using whtever music app you have, and normally in the music player there is a setting to "Set Ringtone" while it is playing.
For music player on my phone, I use the ES Music Player (it comes as part of ES File Manager app).
* I am a Boost Australia customer. This is a peer to peer community where users help each other.
Check your phone's manual on how to put ringtones on your phone. I don't know whether Google Play allows you to use songs from any music catalogue they may have.
I was talking about songs in mp3 format that you may have on your computer or phone. Phone's that can handle mp3's as ringtones, you can select from the options menu of a particular mp3, to set it as a ringtone.
But as I said, your phone's manual should tell you specifically how.
If you use Spotify, then you can use the tool DRmare Spotify Song Converter to help you get Spotify music as ringtone. It will help you download music from Spotify and save as local files. Then you can use Spotify songs on other devices as you want.