If you got a naturally thin frame there's no point becoming a fat bastard. Seriously hit the weights, powerlifting (bench press, dead lifts, squats) is very good, requires minimal equipment, and gives you a great all body workout. There's heaps of suppliments for beginners who want to bulk up, eat alot of complex carbs (pasta, rice, bread), protein (lean meats, eggs, etc).
There's no point trying to stuff up your metabolism and trying to bulk up on junk food, any weight you gain will probably go straight to your gut, your blood pressure, cholesterol levels etc will increase, its all detrimental to your health. Also you'll find its not easy to lose that gut, and you'll still look like a b!tch with your narrow shoulders, thin frame, and big gut hanging out.