If you have a designated "sub out" on a receiver that's all you need, just 1 RCA. Using a 2 channel stereo receiver or amp you will need a left and a right coming from a line level output. I'm not sure what you are running for equipment, if it is older with no "line level out" or "main out" you may have to go with my first suggestion and use speaker cable directly to your sub's high level input, then jump over to your speakers.
Hope that helps...good luck
Hope that helps...good luck