You can use many programs to achieve this conversion, but since you've already got iTunes (you couldn't have bought songs from the iTunes Store without it!), these instructions will cover using iTunes to convert songs from the iTunes Store to MP3.
- Begin by making sure your conversion settings are set to MP3.
- Next, in iTunes, find and select the AAC songs you want to convert to an MP3 by single clicking on them.
- When the songs you want to convert are highlighted, click the File menu in iTunes.
- Select Create New Version and then Create MP3 Version. This will convert the iTunes songs to MP3 files for use on other kinds of MP3 players.
One thing to know, though, is that converting from AAC to iTunes does actually slightly degrade the quality of the file. The reason for this is that both AAC and MP3 are compressed versions of the original song file (raw audio files are very large). Some quality is inevitably lost during the compression that created the original AAC or MP3. Encoding from AAC or MP3 means even more compression as loss of quality. It's so small that you probably won't notice it, but it's true.