How do i connect an mp3 player to my surround sound system through its headphone jack?

You will probably need a mini plug to RCA adapter so that you can plug your audio into the line level input jacks on the back of your surround system.

One thing you'll need to be aware of, is that the audio output of your .mp3 player may over drive the inputs. If that's the case, try to find an adapter that will reduce the volume from your .mp3 player to levels that will allow your sound system to handle the audio levels easier.