you will need a stereo miniplug to RCA adapter. you can find that adapter anywhere... well in electronics stores like radio shack and home entertainment stores like the wiz.
if you want to listen to the stereo on your computer, it's audio out from your stereo to line in on your soundcard.
if you want to hear your computer via the stereo, it's the line out from your soundcard to the audio in on your stereo.