Can anyone help me with gaining weight/muscles? I'm Dangerously thin (Age 17,104 pounds)?

Hi there, My name is Kevin but you can call me Feng. I am Viet/Mandarin/Thai and I'm 17. My birthday is on Jan 24 1991. I really have a problem with gaining weight/muscle. I am so dangerously thin and I weigh 104 pounds. I really really need tips. I often get rejected from the girls I like because of this so it's kinda sad to me to find out that it's because of that. I'm currently a junior and I hope to make a change by college. Arigato/Thank You for your concern >_<