You want to make your own mp3 ringtone from mp3 file, you can use mp3cutter, you can cut what part to become mp3 ringtone. Mp3cutter is free software.

MP3 cutter can extract clips of any length from a MP3 song. These clips can be put as ring tone of the cell phones. If the size of a MP3 file is going out of space,then it can be cut and stored in two CDs by using it. It comes with advanced facilities like display of start point, end point, and length of selection. It can play the selected part of the (clip) song. Includes volume controller.

Version 1.3 is a bug fixing release.