By your description, you seen to have a simple master volume amp and a Stratocaster-style guitar. I will proceed with these assumptions.
There are likely three pickups and a five-position switch, as well as the volume and two tones. The two tones are for the neck and middle pickups, and for now, you can dial them to maximum and leave them there. I will return to the volume knob later.
The idea of a master volume amp is that the Gain controls the amount of distortion you have and the Volume cuts down the volume to acceptable levels. A very clean tone might be Gain at 2 and Volume at 9, while Gain at 9 and Volume at 2 might give a wildly distorted sound at a similar level. I suggest ignoring the amp's Treble knob as well.
There is a "sweet spot" called "edge of breakup", where, when the guitar's volume knob is maxed and you play aggressively, the and sounds distorted, but when you play more lightly and/or turn down the guitar's volume, you play clean. The exact spot where you turn your amp's Gain knob is different on each amp, but it's useful for less aggressive rock playing. Maybe start looking at about 3-5 and move up from there.
For more metal tones, you want more than "edge" but it is likely that you won't find maxed Gain tones useful. The "secret" is that distorted metal tones aren't as distorted as you think they are. Distortion acts as compression, which makes the quiet sounds louder but the loud sounds quieter, so that it sounds about the same when you lightly strum and really dig in. AC/DC tone is lightly distorted but really loud and punchy, for example. Start a little more than halfway on the Gain knob, turning it up (and correspondingly turning the Volume down to keep your friends and neighbors happy) to find the sound you want.
When you get to the ballpark of metal tone, muting the strings you aren't playing becomes a crucial skill, and the guitar's Volume knob will basically be used as an off switch, keeping undesired noises from being amplified.
Best of luck!