your stereo has to have an input line.look for it to say aux. you then need a cable that goes from your sound card to the input on the speaker. you know the end of the computer speaker wire that goes into the computer,well you need a wire that both ends look like that.just go to radio shack and the dude will get you one.
you can also make one yourself.if you have headphones you dont use, you need two of them,cut them and connect them so you'll have 2 ends.