Thanks for sharing your concern. Like our special friends have mentioned above, try not to think or be too conscious about your body. Accept who you are and love yourself.
If you are taking any supplements, leave that boat. Just eat healthy food which contains good protein like fish or eggs if you are a non vegetarian. If not, have plenty of veggies, dal, eggs every morning and evening. And most importantly, exercise well by joining a gym. Many think that only to lose weight people go to the gym. Not true. Even thin people are recommended to go to the gym, which helps people gain weight infact,
Eat, Love yourself and Laugh. Don't bother about people who comment about you.