Here's a link to a site that may help you get started. Looks like you may have to download a plug in to hear the midi files.
I'm sure some surfing would find you some other sites you can sing to without much if any of an investment. This was the first one I ran into.
http://www.ronandowling.com/karaoke/index.php
Here's another one that you might want to check out. Looks like you can have scrolling lyrics with this one.
http://www.comparinifood.it/rainbow/singerneten.html
Good luck,
George