You should provide specs of your setup with your question - more likely to get replies that way. What brand/model of subwoofer and surround sound system are you using? If the subwoofer is not self-powered (i.e., has a wall plug for power) then it will need to be driven by a power amp in the surround system. If the surround system has just an RCA jack then chances are very good it only provides a line output signal, and thus won't run your subwoofer. In that case, you will need to purchase a separate power amplifier to connect between your subwoofer and the RCA line out jack of your surround system.
If on the other hand your surround system does provide a powered output through the RCA jack (some of the cheap systems-in-a-box do that IIRC), then go to Radioshack and get an RCA plug with solder connections, so that you can solder one end of the speaker wire to the plug and leave the other end for connecting to the subwoofer.