The metal guitar sound is treble-heavy, but also thick and chunky. You won't be able to get it out of the distortion on your amp. Look around at different distortion pedals. There are scores of pedals out there, all of which provide different flavors of the metal sound. See which one gives the sound you are seeking. Try combining pedals for an even more distorted sound. Remember that to actually play a gig, you'll need a larger, professional amp. Finally, most metal guitar players drop tune to D, C, or even B.
There are a few techniques to give your guitar playing a tough, heavy sound. Palm muting is the most important. That gives your riffs the "chug" sound that's central to playing most forms of modern metal. Another way to give your guitar playing a meaner sound is to play in down strokes. These just have a thicker sound than playing both down and up strokes. Power chords are preferred over the standard barre chord. Many metal guitarists only play the first two notes of the power chord.
The harmonic minor scale makes your solos darker, with a more "heavy metal" sound. This is an essential scale to know for playing heavy metal. Other scales used frequently in traditional metal are Phrygian, Lydian, Spanish, and Byzantine (the last two are particularly "dark" sounding). These scales are also used in extreme metal genres like death and black metal, with atonal variations. Don't worry about memorizing them all. Pick one or two you like best and get to know them inside and out.
Metal guitarists have to solo fast. Learn how to play your scales, slowly at first, then faster. Select one scale, preferably the harmonic minor scale, which is central to metal soloing. Practice only on this scale until you can really play it fast. Once you've mastered the scale, learn techniques like tapping and sweep picking to increase your soloing speed.