Open iTunes and click on the song you want to shorten. Listen to the song all the way through so you can identify the exact part to edit.
Pause the song at the time you want it to start. Take a note of the exact time. Continue to play the song and pause it where you want it to end. Again, take a note of the exact time.
Right-click on the song. Click on "Get Info," "Options."
Click on the empty box next to the option marked "Start Time." Enter the minute and second you want the song to start at.
Mark the box beside "Stop Time" by clicking on it. Type into the box the minute and second you want the song to end. Click "OK" to shorten the song.