Hi Ramesh, Welcome to HealthcareMagic Forum.
You need to exercise regularly for an Hour daily like cardio, skipping etc. Eat fiber rich fresh fruits and green leafy vegetables. Eat Protein rich foods like lentils, pulses and beans and Skimmed milk. If you eat non veg eat egg whites daily, chicken and fish twice a week. Regular exercise with protein rich diet will increase your Muscle Bulk and exercise will also increase your appetite, helping you eat more. If you drink and smoke, quit them.
Good Luck.

f U R eating well and still do not put on weight, we have to make sure U R not suffering from some disease like Thyroid disease, Tuberculosis some where or diabetes. So please have a complete medical check to rule out any cause. If all are ruled out U must take a healthy protein rich diet and be physically active including some regular exercises daily. A high calorie diet with over 2000 calories, plenty of fruits and salads and proteins in the form of meat/chicken, fish, egg etc & if vegetarian- plenty of milk & milk products,dhal & mother cereals and dry fruits like walnuts, badam etc. Consulting a Dietician for diet will be best.