I cannot gain weight! what should I do?

Im a guy,16 year old. I am very thin and weigh about 53kgs.
I cannot get fat or gain weight. I even try to eat more even ifmy stomach if full but its not helping. The fat is just getting on my stomach region but not on chest or any other body part. I need to gain chest and all. What should i do?
Please help!
I cannot gain weight! what should I do?
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