I assume you are talking about being undermuscled, not underweight.
I'm an ecto hardgainer, as are my 2 brothers. We all grew up skinny with little muscle whether or not we lifted weights or did sports. The solution for us was to wait until our metabolism slowed down a bit and we filled out with some muscle naturally, which happened around age 20. Your super high metabolism will probably prevent you from gaining any fat or muscle for years to come.
If anybody can help a hardgainer put on muscle, it's Anthony Ellis at
Gain weight, muscle building workouts and diets. Mouse over his picture to see what he allegedly accomplished in 3 months. He will tell you to eat a lot and lift heavy. We're talking 5-10k calories, 6-7 meals a day, 150g+ of protein, and supplementing with creatine. You would likely lose the muscle quickly if you ever stopped the routine.
My advice is to lift normally, eat normally, and just give it plenty of time.