Open iTunes on your computer and navigate to your library of songs.
Right-click on the song you wish to shorten, and then click "Get Info" from the popup menu that appears.
Click on the "Options" tab at the top. You will find two check-box options labeled "Start Time" and "Stop Time," each with open fields. These fields refer to the elapsed time the song has played before you want it to start or stop.
Edit the number in those fields to change the length of the song.
Press "OK” to confirm that you wish to change the song to this length. The song will now stay at this length when you burn the song to a CD.