I am skinny. i have 1.55 m and 40 kg what should i do to gain weight?

Talk to your doc: Intentionally gaining weight is not always a good thing. If genetically you are to be in this weight range, and you're eating well, then gaining weight intentionally may put you at health risk. However, if you are underweight, then a thorough checkup and lab exam may be necessary. A dietician can help you safely increase your caloric intake and gain weight.

Check with Dr. : Eat healthy high calorie foods. Eat 6 meals/day. Work on strength training to build your large muscle groups. Workout w heavier weights w fewer repetitions. Sleep at least 8 to 9 hrs/night. Hydrate well w water. Foods to help w healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, extra virgin olive oil, canola oil.