iTunes will convert it for you. Set iTunes in the preferences to import to mp3 files, and then by right-clicking files that you want to convert, you can convert to mp3.
Your next problem is that the music sold on the iTunes music store isn't just AAC music, it's protected with DRM, which means that you can't legally convert it. My advise would be to not buy any music from there at all.
What you can do however is burn the files to an audio CD, then import them into iTunes as either AAC or mp3 (AAC is better).