You just need a lead to connect the line out from your pc to the aux or similar input of your hi fi. Provided your hifi has provision for this facility.
Just to say it's easy peasy really. I've had my mp3's playing through the hi fi for about ten years now. The only snag is if your hi fi doesn't have any input sockets, otherwise it's a simple process of getting a lead with a jack on one end (plugged into sound socket on your PC) and the right connections for your hi fi on the other.