Add a small mixer for the music & the mics.
http://pro-audio.musiciansfriend.com/pr ... sku=630156
Then get a program to rip & play your karaoke songs.
http://www.mtu.com/basics/karaoke-hoster.htm - I personally prefer this
or
http://www.karaokeware.com/CompuHost.html another good program
There are free prgrams out there that others can steer you to & for home use would be more than adequate.
Then for music you can buy cdg's & rip them to your computer and you can also download on a pay per song basis
http://www.tricerasoft.com/cgi-bin/Kara ... /index.asp
www.selectatrack.com
Now it really isn't recommended to run karaoke through a home stereo as live vocals can blow the speakers so you may want to consider a small PA system of some sort or at the very least a speparate set of PA speakers dedicated to karaoke only.