How can I gain weight?

Im a 21 yr old female and im underweight for my age.Im 5'0 so i dont look unhealthy but i would really like to gain about 10 to 15 lbs startin soon.I heard products at health food stores may work but i think it may jus make me muscular and im not trying to be muscular and skinny.So whats the best thing i should do to gain.I only weigh 90 lbs!