If you want to cut mp3 into small parts to be used in mobile phones for ringtones, and you don't have any money to but a small software to cut it for 10-20$, don't worries you can used Mp3 Cutter 1.2, get it here.
First you need to download and install to your PC, it's free, safe and easy to use. Read this following instructions :
Then try to run it under Start -> Programs -> Mp3 Cutter, it will shows just like this picture :
Try to Click open to browse your mp3 song to be split into small part, see this picture :
Then click "Set Start" button to start song to be cut of and after that click set end to stop the song to be cut of. Then after you have finished save your song with "Save Selection" button and your song are ready to be transfered into your phones.