What amp do you have? What head unit? What is the amp hooked up to?
The easiest way is this (not an exact method, but close enough and on the safe side); on your head unit volume, find the 3/4 setting (3/4 of the highest it will go). Then turn the gain on the amp, all the way DOWN. Now start the car and turn on the radio, move the volume up to the 3/4 mark. If its already louder than you will ever listen to it, leave the gain alone, if you want a little more volume turn the gain up some (just a little). Do this until you get a volume that is as loud as you normally listen to music.
The point is to match the amps gain to the head so that they clip at the same time. This gives you the most power for an acceptable noise floor. Doing this is somewhat complex (depending on how many amps, processors and such), and the method I gave will give you a 'ballpark' setting.
