Phew. if it was an m4p tack you bought from itunes you'd first have to burn it to CD and then follow the instructions below.
First step is to rip the track to an AIFF file (you can do this just by dragging it from the CD displayed on your desktop to your hard drive). Then just use Audacity, or something smilar including GarageBand, to trim the AIFF file.
If there is a simple solution, you can count on the Apple developpers: they have implemented it. The hardest part of working on a Mac is to find out that simple and easy solution. And sometimes I have the impression, by working on Windows, you slowly lose the ability to find that straight and easy path. 😀