what sub-woofer should i get with my receiver

You will need a powered subwoofer for that. These have their own separate amplifier and usually a standard wall plug with a single RCA line input.
 
Just by looking at the picture of the Klipsch SW, apparently you hook it up using standard left and right speaker out (zip wire or Monster cable, whateva) from the rcvr or use the L&R line out jacks from rcvr to SW and can hook up speakers from the sw or the receiver.
 


my receiver only has a preout for the sub what do i use to hook my sub up to it
 
Take the single subwoofer output from the receiver and buy a rca splitter and plug it into the red and white rca's on the back of the sub.
 


what plug do i need for the reciver?