It is recommended that a teenager should get one hour of physical activity mostly each day. For a grade 8 student you should run 4-6 miles per day and gradually increase, also you should take a few breaks in between days. Doing cardio for example push ups, pull ups, sit ups, and many more is important to do at least 3-4 times a week. Eventually you will increase the amount of cardio you do as you keep doing it. You can start by doing 1 set of 15 reps and increase the amount of sets and reps. When you increase the amount of reps and sets make sure you don't increase them to fast or by too much because you don't want to injure yourself.