How do I convert AAC files to MP3 using iTunes?

In iTunes, click on the Edit menu and select Preferences. On the General tab, click Import Settings. Change the "Import using" setting to "MP3 Encoder." Save those settings and you"ll then be able to right-click on songs to convert them to MP3.

Note: if you purchased the songs from iTunes, some or all of them could be copy-protected and you won't be able to convert those to MP3.