You can easily put that mp3 directly into Garageband. Simply open GB, under the Track pulldown menu select New, and drag the mp3 directly from your desktop or iTunes or wherever onto the track.
Highlight the track by clicking on it once, skim through it by moving the playhead wherever you want, and when you find a place you want to make a cut, position the playhead over it, and press apple T.
Easiest way IMHO. Sometimes though, when you transfer the mp3 back into iTunes, the mixdown can create some sound distortion.