You need 4 wires going to your amp.
Power = this usually comes from your battery. It is always on
Ground = this is usually attached to your car chassis. make sure you have a good ground....sand any paint on the metal and make good contact.
Remote = this is usually the blue wire behind your radio. You need to connect it behind the radio and run it to the amplifier. It is usually a thinner wire..usually 18 gauge give or take. It will be smaller then your power/ground wire.
Rca = this comes from the back of your radio to the amp. many times you can run your rca/remote wire together....some do not recommend it. Up to you.
The crossover is what stops certain frequencies going to your speakers. Usually subwoofers like anything below 80hz (give or take)
So you can adjust your crossover to "lowpass" and turn the dial to 80hz as an example. This will help the speaker only play things under 80hz so to speak.