I personally use Audacity on my PC to do the work. In Audacity you select a portion of music, copy it to a new track, then save it in MP3 format. I'm fairly sure most ringtone creation tools on the Play Store work in a similar manner.
I moved your post to a new thread, as we have a rule on this forum regarding resurrection of threads that have been inactive for a long time - six months or more. In the future, make sure to check the date of the last post before responding, but please feel free to create a new thread rather than responding to an existing one in cases like this.