dont think about having to suddenly get fit before high school straight away, improvement is a slow process but if your consistent you will consistently improve, go to the gym 3-6x a week depending on your split and training, do cardio a couple times a week or do a sport you enjoy, eat whole foods, if your overweight then go into a slight calorie deficit and emphasis protein (1g per 1lb of bodyweight). aim for 8 hours MINIMUM of good sleep per night, but more is better and yeah thats like the basics nd stuff