[header] How to get in shape as a teen [title] Start slowly and build up. [step] Try to take a ten minute walk every day after school. Add a minute every day until you're walking for an hour every day. 
If you don't have a safe place to take a walk, try climbing stairs instead. Walk up and down the stairs five times on the first day, and then add one more flight of stairs every day until you can go up and down twenty times without stopping.