Unusually light for my age?

I am a 16 year old guy, I am 5'11, and I'm only 120 lbs... Pretty much every other 16 year old guy I know is between 140-170 lbs. I don't like how lightweight I am, and so I've tried putting on weight but it just doesn't work. I gain maybe 5 lbs but then lose it within a week, which is surprising to me because I never work out & am incredibly lazy. Is there any way I can gain weight & keep it? I dislike how light I am, & would very much like to gain SOME weight...