You need an amp that runs whatever the woofer can handle at whatever the resistance of the voice coils of the sub are. so if your sub runs 200w RMS(continuouspower) @ 4 ohms resistance, then you need an amp that runs as close to 200wRMS at 4ohms. It's ok to be a little under or a little over. the general rule is that the power the amp puts out should be in the top 1/3 of the subs handling capacity. Info on your specific sub could probably be found on crutchfeild or any other car electronics site.
Keep in mind things arent this simple when using more than one speaker.