You just need a conversion program. Here are a couple:
In newer versions of iTunes, you click on a song in iTunes, go to the top menu that gives you the option to "convert this song to MP3." That option might say "convert this song to AAC" in that case go to your preferences in iTunes, and choose your preferred conversion is MP3 (not AAC). From that point on you can convert all your files to MP3 if you wish. You might not be able to convert songs with extension M4P; those are iTunes purchased protected files. You need to call Apple and ask how you can convert those, but an easy workaround is to burn an audio CD with all the protected files; then insert the disc into your computer and convert them to MP3! Good luck!