Gaining

Graze throughout the day on high calorie snacks..savoury or sweet as you prefer.
Eat mainly each evening,focussing on a lot of carbs such as bread and pasta..build up gradually,eating just a little more each night and wash it down with a shake with cream.
Don't get too hung up on it..just eat and don't expect results..enjoy the food.If you eat what you like and want,you'll eat more.

Get old!
Seriously, I had a fast metabolism when I was your age - weighed about 150 despite doing no great exercise and eating junk. Once I hit 24/25 the weight just trickled on. Without trying or really wanting to I end up at 225 so it will get better