How to gain weight??

Hello hello!!

So I have a question, since I can't really quite find a right answer.

I'm 15 years old and I've always had a fast metabolism. I can eat what I want and I won't gain ANY weight.

Does anyone know how to slowly gain weight in a HEALTHY way?? All tips are welcome!!

I'm 171cm tall and 55kg!

(5"6 & 121 pounds)