http://www.apple.com/itunes/download/
Download and install. Open it up, edit->preferences->advanced->importing->Import using (select "MP3 encoder")->Setting (select your choice, I usually have 128 because any higher and I can't usually tell the difference)->Ok.
Find the music tracks in windows explorer and then drag and drop them into your iTunes library->Right click them on iTunes->Convert selection to MP3->Right click them on iTunes->Show in Windows Explorer.
Remember where it is and which file it is because that's what you'll want when uploading to MySpace. If it's easier, copy and paste (or cut and paste) it to your desktop.