concerning the guitar, I would suggest this tutorial from Christopher
http://www.guitartricks.com/tutorial.php?input=365
In it, Christophers explains the different parts an electric guitar and should answer many of your question concerning the knobs. Altho he uses a stratocaster, the basic principles stays the same.
I have never seen that type of Marshall but what are the labels of the different knobs?
Normally you'll have something like Master Volume, Gain, Low, mid, treble.
Very rough explanations without going in details...
Master Volume : let's you control the loudness
Gain : The more gain, the more distortion you should have
Low : Controls lower frequency, too much low will make you sound muddy, too little will make it bright
Treble : controls higher frequency, if you would turn this way up, it would start to sound scratchy and sharp. The less treble, the less clarity you'll have.
Mid : Controls everything between low and treble. This is often where you'll spend most of time working on :) at least that's what I do. Mid range frequency are to me the fine tunings of my tone.