How to connect a pc subwoofer to a home theater receiver?

[Lisa]

I have a new Sony receiver with a RCA output for the subwoofer. Subwoofer must be powered. I am using my old speakers with the new receiver and my subwoofer is passive. My step dad gave me the subwoofer that went to his pc surround sound set. The sub input on it is a mini-headphone type plug. I think that is called 3.5m plug? I had a Y RCA to mini headphone cable lying around and connected one of the RCA plugs into the receiver output and the mini headphone plug into the sub woofer. Nothing. There is a "socket" of sorts on on the back of the subwoofer that says "volume"...no knob, nothing. My step dad didn't have any other cables. Did I hook it up incorrectly or do I need more cables? Or is it just not possible?