If you purchased the tune from itunes it will be in one of 2 types of files. right click (on PC) or control click (on Mac) and click "get info" That will tell you the file type. They normally are m4a or m4p. You can convert those however to mp3 in itunes. Right click or control click (mac) the song and select "Create MP3 version"

When you rip tunes in itunes you can specify the file format also. Under iTunes/preferences/general/import settings.