Mate, you seem REALLY skinny.
Though, this COULD be due to your metabolism, so, I wouldn't worry about it TOO much yet.
Give it a couple of years because your metabolism tends to slow down when you 18/19 (did for me) and I started gaining weight.
I am now nearly 20, 5'7 and weigh 140lbs (10 stone).
I eat healthily enough and I guarantee you I eat my 5 a day, enough fat etc...
Don't worry about it too much.
ALTHOUGH, is your diet strict or are you a little flexible, i.e. do you snack?
If you want to gain weight, I would recommend building muscle mass but, like I said before it is probably a high metabolism.