I do this with iTunes.
First, in the iTunes preferences make sure that your importing preferences are set to MP3 with an appropriate bitrate for your phone.
Then right click on the song, click get info, and select options. Set the stop time to what you want. Finally right click on the song again and click convert to MP3.
That'll generate a new MP3 of the song that's the length you wanted. You can then change the end time back for the original song, transfer the new one, and then delete it from iTunes.
That's probably a little more complicated then garage band, but I don't have it installed, and this was the simplest way without downloading anything.