You'll fill out, dude. Don't go to any great lengths to try to change your body type. You'll just get fat. Go to the gym, check around online for some good weight lifting routines, and eat healthy. Those crazy diets where you drink 3000 calorie milkshakes or eat a ton of food you wouldn't normally eat are terrible for you. Just be sure you're eating quality foods (cook your own meals when you can) and be sure you're getting good portions of proteins -- soy, fish, chicken, and the occasional red meat. And there's nothing wrong with being skinny as long as you're in shape and not unhealthy. Trust me, whoever you're interested in will dig it as you get older.