If your Mac is less than a year or two old, the headphone jack on the side is dual function: it’s both an optical digital output and a stereo analog output.

If you go analog, you need a 1/8″ stereo to RCA stereo audio cable, available at any Radio Shack or electronics store (but see below for a better price).

If you go digital (and if your receiver has an optical audio input), you need a mini-toslink to toslink cable, and these are hard to find in stores (but see below for a great price). The advantage to this is that you will be able to use FrontRow or Plex to play movies with full surround sound.