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...