How to become fat?

my age is 17 and iam so thin that dresses does not fit perfect giv me suggestion to become fat

Buy your clothes to fit your body, don't change your body to fit in clothes.
You have to buy clothes specifically for petite people. If you are having problems in the chest area, spend a little money on a decent bra (have it fitted specifically to you). If you eat healthy and you are still super skinny, consider yourself lucky.

well it depends on stuff like if your naturally thin, hen it maybe a bit harder to gain weight. but theres a lot of things like eating the right foods...you dont just eat more food like people say, you eat a normal diet with breakfast, lunch an dinner ect but you can add things in there with more calories to gain weight.

Donuts, MCDS, carbohydrates, but not all the time of course, butter... those type of things

EDIT: I'm so sorry, you are a boy and don't need a decent bra. but my advice still stands, there are clothes that are tailored to small bodies. You just have to look and you might have to pay more for them.