Please some advice on what to eat, what sports to do...I'm lost

I am 20.

Always been thin ..maybe a bit too much. I want to gain weight.

People I know don't take me seriously and say "oh why you complain?? At least you don't have to lose weight, THAT is hard..."

No, gaining weight can be hard too for some people.

So I try to eat more, I also eat more junk food (which is no good I know but it's efficient 😅 I can't eat more healthy food...so I eat as usual but junk food)

I also like sports.

I mostly dostairs, rowing machine. And I do squats every mornings, so I work my legs and butt pretty much.

But I feel like it increases my cellulite which is really not my goal😅

I guess it's because of junk food...

How can I gain weight while getting rid off cellulite??