Hi Rency,
To gain weight ,we need to do more of weight training exercises. If you have the misconception that gaining muscle weight will lead to give your body a muscular look then its not true. Boys build up muscle because they have male hormones which put them to build huge muscles. Otherwise it wont be the same with girls. Just you need to do weight training exercises for 3/week and do less counts with more weights. Also, include more of carbohydrates and protenious food so as to provide your body with lots of calories.