Setting up your amp ....

First of all you should have a couple of knobs on your amp called HPF and LPF ... These stand for High Pass Filter and Low Pass Filter 

A HPF will only allow the High frequency sound through ... Basically if you have speakers or tweeters and you want to cut out the bass (because it makes them distort)

A LPF will only allow the Low frequency sound through ... Basically if you are using it for a Subwoofer

And NORMAL will play everything.

If You are using an amp for a SUBWOOFER set the amp to LPF on the knob there will be 2 or more numbers around the outside, usually from 20Hz - 250Hz turn the frequency to as high as it will go This is the cutoff frequency, so if you turn it LOW to say 20Hz the amp will play any sound from 0Hz - 20Hz if you turn it to 250Hz it will play any sound from 0HZ - 250Hz turn it to the highest to start with and then just mess around with it and see where you like it best .... Every setup is different so there is no definitive answer .... generally the bigger the sub the lower it needs to be.

The HPF is the completer opposite, althought it is hardly ever used for setting up tweeters which are connected to an amp. Usually the HPF is used to control the amount of Bass which it is allowed to play. My amp for example has a HPF from 50Hz to 250Hz ... The HPF is best used if you want to amp small speakers such as door speakers which are typically 17cm or smaller. The Hz settings tells you he lowest frequency it will allow through, set to 50Hz it will play any sound 50Hz or higher, set to 250Hz it will play any sound 250Hz or higher .... Dont go setting it to 250Hz straight away and expecting your subs to handle all the bass ... a surprising amount of bass is through normal speakers, set it to as low as it will go and then try it, usually this will be far too low but as before, have a play around and see what works best for your setup.

If you use the setting in he middle, usually called full it will play all parts of the song, both low and high, usually best for 6x9s which can handle bass and treble

LEVEL

Basically .. Use it as a volume control