You cant. iTunes are DRMed with a proprietary format that nothing other than the itune player or ipod can play. Which is why I do not touch anything apple.
Apple does offer some DRM-free tunes, but you (used to, anyway) have to pay a bit more for them - I think $1.29 or something. If you have any like that, then you can convert them I think. But I would guess the majority are in the proprietary, DRM format.
Burn them to an audio CD with iTunes...then rip them back again to a WAV or MP3 format. Preferably WAV if you have the file space because compressing to MP3 will just degrade the sound quality even more.