Converting to mp3 is easy if you ripped the songs from a CD:
1.Make a play list with the tracks you want to convert.
2.Select all the tracks then go to the ‘advanced’ menu and select ‘convert selection to mp3″. The files will convert – so you’ll have duplicate versions of the same songs in your iTunes but you can delete them once you’re done with them.

If the files were downloaded from iTunes and have DRM copy protection, the easiest way is to burn them to CD and then re-import them into iTunes with your import settings set to mp3. Those can be changed by going into your iTunes Preferences, click on the ‘advanced’ tab, and then the ‘importing’ tab.

Hope this helps!