load a song into garageband or Quicktime pro, trim it, then save.
Garageband will save as AAC or Mp3 I think, most phones can support MP3. Quicktime Pro will save it as AIFF, just import it into iTunes, then convert it to Mp3 and delete the original.
AIFF is just the Apple version of WAV -- they are uncompressed and very large files compared to the length of the song.