Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
iTunes. You edit the length of the song and convert it to AAC--all in the program. Personally, I prefer using audacity to edit my mp3 down to 20-30sec size, but that's just me.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.