How to convert a big mp3 file to a smaller one?

i have a song (an mp3 file) that is 11 MB, and my brother has the same song, same quality and it's only 5 MB. my ipod just happens to have a smaller memory than his but he doesnt want to share the file with me. does anybody know how to convert it to a smaller mp3 file? or if it's a program, can anybody give me the download link for a freeware?