This article explains how to use iTunes to create MP3s and AACs from your CDs. It also includes information on how to change the import settings in iTunes.
How to Convert AAC Files to MP3
Each file type has its own strengths and weaknesses: some have higher sound quality, and others create smaller files. To take advantage of different file types, change your iTunes import settings.
Open iTunes and go to Preferences:
On the General tab, click Import Settings.