How can i gain weight ? im 22 5'1 and weigh 98 lbs

BMI 18.5: We use body mass index (BMI) to use as a reference point when discussing appropriate weight for height. Check out http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm where 18.5 to 24.9 is considered normal. Gaining weight is same as losing it, except in reverse. You still need to eat clean, healthy food, plenty of fruits & veggies, plus protein from various sources, just more of it