Don't worry about weight at this age, it will gain muscle since you are growing. But still if you want to gain attractive body then you can start workout at home or in gym. Better to join gym where trainer can guide you for diet and workout.
Along with exercise at home , start increase your diet slowly. Muscles and fats grow together that's reason you have to exercise to reduce fats. Otherwise your physical structure become ugly e.g. increase in belly size, double chin , increase in thigh size.
Health powder - Instead go for natural food like Chicken, eggs, fish, soya. Same content you will find in health body gain supplements. 'ON' company products are famous in India but you can walk to any sport store and ask about it. Before purchase always do research about pro and cons of that product.
Exercises - Muscles increased by hard workout which is only possible in Gym since they have those equipment. If you wanted to only stay fit with average workout then buy weight dumbbells, Bar , Bar plates at home. Have daily run in early morning, With little warm-up prepare schedule to do exercises for upper and lower boy. Perform crunches for abdomen alternate days. Check out youtube videos or browse online for each type exercise and benefits of it.
Medicines - Please avoid this. It is not recommended at all. Consult doctor before you plan for medicines for weight gain.
There are little percent of hereditary impact on your body structure too. So if you parents have healthy body with good weight then definitely you will also gain weight down the line.
In nutshell proper diet with exercise will increase both muscles and good body structure.
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