Easy
Ok, find your speaker port. The one with with the little speaker on it, that you would normally hook up computer speakers to.
Then, go to an electronics store and get a Mini to RCA cable, they usually run about 4 bucks. Make sure you get the stereo model, and not the Mono.
Then, plug that into the back of your mac. It will split the audio into 2 cables, and then you can hook up your speakers via rca cables. Quite easy.
If you want to have sound on both, you will need a splitter, its like Mini>2 Mini.
Mini is that speaker port.