First we must know what is you height to decide about your ideal weight your weight is 55kg so your height could be 5 feet to have BMi at 25 but if your height is more than 5 feet and 6 imches you may be mildly under weight. Take a balanced diet with breakfast lunch and dinner combination of protein carbohydrates and fats fresh fruits milk and milk products . Most inportant physical activity play some games outdoor or join a Gym where you can do some weght training also have patientce you will gain weght but if there is no response see a endocrinologist to check of hormonal problem eg thyroid or other problems.