It really does depend on what YOU like. There are a few things - like for a clean sound you'll want the gain low, and if you have distortion on the amp, turn it down. Treble and Bass you'll understand way more by trying them out - I couldn't put it in to words that would make any more sense.

Try playing with the treble all the way up, and bass all the way down, and then vice versa - you'll see what they do. Guitar players as a rule play with everything dialled to 10, or maybe have the 'middle' low to get a metallica type scooped sound.

Seriously, you'll learn way more about your amp if you just go and play around with it. You'll eventually get a sound you like, and quite possibly never touch the controls again!

What amp you got dude?

:D