You should ingest more energy in form of more calories with good quality proteins and nutritious food on daily basis than you use up for physical activity. Add 500 calories per day to your current requirement. Eat more. Eat 5-6 meals per day. Avoid skipping any meal. Increase your portion. If you eat 2 chapattis, then have 3rd one. Eat corn,potatoes, peas,banana,custard apple,chikku,mango,peanut butter,cheese, milk with 2 tsp skimmed milk powder, boiled eggs. Maintain 2 hour gap between each meal. Drink plenty of water,juices,milk shakes,smoothies. Exercise daily. Exercise will stimulate your appetite,increase your energy needs and will convert extra calories into muscle. Sleeping is the best thing you could do to keep yourself healthy. Take afternoon nap immediately after lunch. Make sure you sleep for atleast 7-8 hours every day at night. Do let me know if you require diet chart plan for weight gain. consult privately for customized diet plan, definitely it will help. Take care

Hi protein rich food like soya , cereals , pulses , egg , meat would build up muscle mass . and regular exercise would keep you fit . not to go for oily cheesy junk foods aas it results in unhealthy weight gain.