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.

Eat healthy foods: high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8 - 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars, lean meats, poultry, fish, smoothies