How can I gain weight?

Okay,I have a problem.Ever since I was about 12 or 13 years old,I've been trying to gain weight.Im 5'9",15 soon to be 16,and weigh 110-115 lbs(It depends on what weight thingy I use to weigh myself).Im tired of people calling me anorexic,or saying that I don't eat at all.I eat CONSTANTLY!If my parents let me,I would eat us out of home.When I go to restraunts with buffets,I can eat 3 or 4 plates of food,depending on how much I eat on each plate.Especially when we have junk food at home.I can eat 2 BIG bowls of ice cream,especially if it's chocolate flavored.If we candy like gummy candies,I can eat all of those up to if my parents let me.I eat the second most in my family.I come right after my dad. XD Is there anyway to gain weight so people can stop telling me that im anorexic?It really ticks me off when they say that! >.<