You are too young to take medicine for your issue. Your body is still developing and you will eventually be able to gain weight and muscle once growth is complete.
Right now you need to follow a proper diet to gain weight and muscles.
Start adding protein go your diet in form of
-Milk and it's products like curd, paneer, buttermilk
- lentils (Dals)
- Sprouts (like moong, matki etc)
- Pulses (like rajma, Chole etc)
- Egg
- Seafood
- Lean meat
Include fresh fruits and veggies in your diet
Start eating small but frequent meals.
Avoid junk food and large meals
Eat home cooked nutritious meals
Next Steps
Follow a 6- 8 meal pattern.
Get adequate sleep (7-8 hours of uninterrupted sleep at night) as muscle building is dependent on your sleep quality and quantity
Health Tips
For a detailed diet plan you can contact me online via practo consult