I am underweight, how can i gain weight?

What do you want?: The last thing you want to do is garbage up eating greasy, sugary stuff. Your best move is to hit the gym circuit, both resistance and aerobic training. "underweight" is by itself not a problem -- the athletes in the photo are probably "underweight" -- except as a marker for illness or poor muscular development. Your physician is your best guide here. Best wishes.

Eat healthy foods: that are 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 to 9 hrs/night. Hydrate well w water. Foods to help w healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, Avocadoes, granola, trail mix, dried fruits, extra virgin olive oil &, canola oil.