You have a few options.. Either a simple 3.5mm Mini jack to phono(rca) connector cable. These are readily available and cheap. Mini jack goes in the macs headphone socket. phono/rca cables go to the inputs in the back of your hifi/receiver. May need a long cable though...
Or, much the same thing, but via a FireWire or USB audio interface. You'd only do this if you are fussy about sound quality. Modern macs built in outputs are not all that, so many people prefer to go via a decent audio interface first. Which interface? Theres a multitude of them to choose from. From cheap to very expensive. The choice is yours.
Or, finally.. A semi wireless system. Send music wirelessly from iTunes to an Airport express. You'd still need the cable I described earlier, to go from the Express to your hifi/receiver. But its a much more elegant solution 🙂