I generally use a clean channel on an amp, whenever possible, and use pedals/multi-effects to create the distortion and/or other effects. On the amp's EQ I will usually have the treble set to just over half way, the mid set to approximately half way, and the bass just below half way.. I have my reverb setting on anything from 4 to 7 (out of 10)..

but like strat-man said.. the sound you want is really an individual thing. So the settings I just outlined then may not be what you want.. I used to have my mid set all the way down, my bass all the way up and my treble around the 75% mark.. but that was when I used to use the overdrive channel of the amp..

There's so many different tones and styles that you could possibly use an amp for.. but the general rule of thumb I have found is to start with everything on a neutral setting. So treble, mid and bass set to half way, and the reverb to about 3 or 4.. then you tweak it from there..