How can I gain weight?

I'm 16 years old and weigh 107 pounds and I'm 6ft. My diet mainly consists of fast food like double quarter pounders and stuff, and some home cooked food like chicken on the grill, steak, etc. I dont drink very much water and I drink lots of soda, like 9 cans a day and some 20oz. bottles and mabye even some with fast food meals. As far as breakfeast I just eat cerial. I never eat lunch during the week unless I'm not at school. I'm not very active I skate and thats about it. Despite all of this I never seem to gain weight and it can't be healthy eating stuff like double quarter pounders everday. Please help thanks (: