"Ringtone maker" is the best app ever which allow you cut song and create ringtone in few steps.
How to use
1) Open app and select Mp3/Audio Cutter Button.
2) you will see full song list choose song from list. here you can search also.
3) choose seconds or drag handle
and select part of song which u want cut. and click save button and your ringtone is ready.
This app is base on Ringdroid code, and licensed under the Apache License.
Ringdroid code: http://code.google.com/p/ringdroid/
Apache License, Version 2.0: http://www.apache.org/licenses/LICENSE-2.0.html