Hi Garima,
To loose fat and gain muscle mass, you need to have a balanced programme of healthy diet and regular exercises. Do 45 min of cardio activities like brisk walking, jogging, running , etc while include alternate days of hard core weight training exercises to build muscle mass. Cardio activities would help to normalize down your body fat percentage while weight training exercises would help to increase the muscle mass. Also, avoid junk food items and late night dinners as our metabolism slows down during night.