I m 23 yrs old. my height is 181 cms nd weight is 68 kgs....how can i gain fat without medicines?

See below: Why do you want to gain fat? Your body mass index (bmi) is 21, which is in the normal range (18-25). Extra fat would increase your risk of heart disease as well as of diabetes, not to mention putting more stress on feet, ankles, knees, & hips. Adding lean muscle mass makes more sense.

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