How can i gain weight?

Hi there,
I'm a 6 foot 1 17 year old girl. Im struggling to gain wait. At the moment i weigh 9 stone 2 pounds. Apparently this is underweight for my height and age. I used to be ten and a half stone but the weight just seemed to drop of. I don't like my appearance at the moment as i feel i look underweight. Ive tried eating more and not exercising but my weight wont go up. I don't have a balanced diet, as i don't really like eating healthy food, but i dont think thats effecting my weight. I find since i lost the weight that i get faint, and feel light headed. Any ideas how to fix this?
Thanks.

[Barb135]

If you get faint and feel light headed, I'd suggest that you might have a medical issue causing you to lose/not gain weight.  There are a couple that come to mind right away.  One is a thyroid condition called "hyperthyroidism", in which your thyroid gland produces too much hormone and speeds up your metabolism, which will cause weight loss.  The other thing is diabetes, which can also cause weight loss, along with other symptoms -- particularly, excess thirst, then having to use the bathroom.  

The faint, light headed feeling could also be caused by too low blood pressure.  

If you have no medical issues, you might drinking something like Boost or Ensure to help make sure you get vitamins/minerals you need since you say you don't eat well.  

If you can give an idea of things you enjoy eating, maybe we can help come up with something equivalent that you might like also, but would be good for you.  

Another thing is that you need to exercise in order to keep your muscle mass from breaking down.