I am 21 with height of 170cm but I weigh only 48 kg and iam so skinny what should I do to improve my weight? What proper diet must I follow?

Diet and muscle mass. You need lean muscle weight gain. In general to gain weight you need to consume more calories than you burn. Do this in a healthy way. Don't choose junk food. Make sure you get all of your essential nutrients and Amino Acids in calorie-rich food. If you do this in conjunction with weight lifting and other similar resistance training, you can increase your muscle mass.