Assuming you would like a tutorial on how to make a YouTube song your ringtone on Android:
First, open the YouTube app and find the desired song. Tap on the three dots in the top right corner of the screen and select Share. A list of sharing options will appear; choose Copy link.
Next, open your web browser and go to Myxer.com. Type or paste the copied YouTube link into the search bar at the top of the screen and press enter. On the next page, select MP3 under Choose your format and tap Create Ringtone. Once finished processing, tap Download beside your new ringtone.
Finally, open your phone's Settings app and go to Sound > Phone ringtone. Tap Add from device storage, then navigate to and select your new ringtone. Press OK when prompted, then exit out of Settings. Your new ringtone will now play whenever someone calls you!