If you smoke, quit smoking. I was really underweight a couple of years ago, and smoking a pack a day. When I quit I gained a fair amount, as my apetite increased. I'm 5'10, and when I was 20 I was about 125 lbs, and now I'm 160 lbs and 24. I gained 20 lbs in a matter of a couple of months, then stabalised for a while at 145 lbs, and then again I gained 15 lbs to where I'm at now.
You are 18, so you'll find when you get to your early 20's you'll gain more easily. But like has been said above, lift weights, eat well and you should be fine.
"it's kind of like an Airbus, it's an engineering marvel, but there's no sense of passion" -- J. Clarkson re: Coxster