Gaining weight but have fast metabolism?

I am 18 yrs old about 140 an 6'1 Im trying to gain weight so that I can look bigger I go to the gym 6 times a week also. But my problem is. Is that I have a fast metabolism an idk how to gain weight, Im pretty much a food disposal too but I can never gain weight. Does anybody know any supplements I should take or go on any diet?