Either Join a Gym and start doing some weight training. Also remember to take a lot of protein suppliments and protein rich food
OR
Have heavy meals(incl junk foods) and sit infront of your TV or computer and don't even shake a leg unless you really have to
Both are equally effective on GAINING weight
I prefer the First one BTW
Eat lots and workout.
Your food should be clean and healthy or you will just get a larger stomach.
And diet is more important than workout.
I was the exact same age, weight and height when I started with a diet plan(Now I'm 19.5yo and 64Kg).
Hostel !! i think it has also a role in my low weight. I'll be there on 31they screwed down my weight 0.5kg Previous month.
If i have extra fat then i think i could survive left 3 years.