a subwoofer is primarily for bass. if you hook it to a mono block amp you will get nothing but bass out of your sub. with most 2 channel amps you have an option...HPF(high pass filter) and LPF( low pass filter) you would want to set it to LPF. this allows the sub to only pick up the low notes( bass notes) the HPF is designed to run mid range or full range speakers.
theres really no way to stop these notes from going to the front speakers but they are designed to play a large range of frequencies, high and low. but the sub should over power (be louder) then your full range speakers so the only thing comming out of the sub is bass.
if you have everything set up, check the frequency on the amp. if a crossover is being used, set it to around 80hz and the same with the amp.