I M now 15 years Old and my Weight is still 35 kgs, Please help me to increase weight & look smart & Healthy!?

I M 15 Years Old Guy My height is good my relatives like My growing Height My Face & Body Comlection is good But the One & the biggest issue is of my weight that is still 35 everyone, my friends, cousins, siblings, even my Teachers and my relatives make fun of my thin & slim body so plz give me a Health Advice to gain weight and look smart like others!