Exercise, an increased intake,healthy high carbo diet, and possibly some weight gain supliments, but check with your doctor first on those.
You don't really want to get fat - you want bulk, and the best kind is from muscle.
Like me you probably have a high metabolism and so your body burns straight through anything you eat. However, if you are developing muscles through the right excercises, your body will begin to divert and balance your caloric intake to help build them.
Lemme put it this way - ever see a skinny bodybuilder? Sure they may not have your metabolism or be starting from your weight, but you won't stay skinny with muscle mass!