If you look close at the plugs for the speakers on the back of your computer, there should be a small imprint in the metal above or somewhere near each plug. You may need a flashlight to see them. One will look like a small microphone, so naturally that's where the microphone goes. The others will have something resembling a ball with wave-type shapes going out away from them. Plug the speakers into there.
If you don't have these imprints, just plug away until you find which one works