High metabolism, Can't gain weight?

I'm like 126lbs 15 years old im in the 9th grade im 5'10. I have a extremely high metabolism, my parents had me when they were skinny, my dad is in shape now but my mom was skinny and my dad was lanky also.. Both of their metabolism's have gone down they are about 40. How do i make mine go down now so i dont look skinny for nearly the rest of my life. (i said my parents were skinny when they had me due to hereditary reasons) and how can i gain weight? im in weightlifting, its somewhat helping... not really though, ive been in it for about. 1 month.. every day, lifting.. 5 times a week, ive gained about 1-2 pounds..