Finding it hard to gain weight?

i turned 17 last month and am really concerned about my weight, i'm 5ft 8 and weighs about 120 lbs + i look very skinny
no matter how much i eat i dont seemed to be gaining any weight, i usually have a balanced diet but thats not helping at all, and i started having loads of sweets and snacks since last november and that didnt have any effect on my weight.. so it'll be really helpful if you can give me some tips on gaining weight