What to do regarding my too low weight ?

Ok so I am a boy and a bit frustrated. I have all the luxuries of life, but my weight. Boys of my age have good bodies by default. But me, not so lucky. I am 21 years old and my Height is 5 feet and 9 inches, which I consider to be good enough for me. But just look at my weight, I am just 49 to 50 kgs. I am very thin and look lean. What to do about this ? I take a proper diet most of the times and eat somewhat healthy food. I even joined a gym, I noted a bit of improvement but gave it up because of my laziness. Whats the best way to gain weight in a short span of time ? I cant gather enough confidence while speaking to people because of this. If anyone can suggest something, I shall be forever grateful to them..!