A good, simple fitness plan for teenager ?

My mates 15 and used to be very sporty, but since about a year she's become more lazy lol and also moved away from her old school etc and hardly does any exercise. She wants to become super-fit again but knows that it'll take time to get back to the same level of fitness again. She used to do boxing, netball, rounders and athletics before. She'd like to know how to slowly build up her fitness with simple exercises like jogging etc so that she can be fitter to join the army and pursue her ambitions of a sports coach (: