Always make sure you are physically active. You cannot sleep 12 hours a day for 365 days and expect to be physically fit. Get outside of your parent's house and do some walking while you are still 15 years of age. You cannot be physically mobile if you lying on your couch and always eating fatty foods which will in the long run make your health really poor. Perform some vigorous exercises on a daily basis to keep your muscles, bones and joints strong. Always lift a few weights to keep the body in great shape. Perform 100 push-ups a day and in 365 days you will see improvements in the way you physically appear.
Eat a lot of healthy foods and avoid unhealthy foods during your teenage years. Eating healthy foods will prevent you from growing fat and physically unfit. The more fats you can lose from the body, the more physically fit you may get. As a teenager, you need to eat the right foods. Focus on eating vegetables and limit the amount of meats you eat on a daily basis. Both vegetables and meats give you proteins which are vital nutrients but of course vegetables are slightly healthier than meats. You don't want to be working out everyday and eating unhealthy foods because you may never gain the physical fitness you want if that is what you are doing now.
Play physical sports like soccer, basketball and football. Teenagers who take part in sports like soccer and football are physically strong and can run for minutes tirelessly. Play soccer if you want to develop some good-looking body parts because it is the most physical sport amongst the rest.
If your already skinny and want to gain strength, do not worry as much about the foods you eat, just work muscles and continually do the 100 push ups a day with preferrably over 300 sit ups. (20Push-ups 5 times, 30 sit-ups 10 times.) Free weights are always better than designed ones also!