Fitness tips for a thirteen year old?

I am 13, 5ft 7, about 7 1/2 stone. I have a reasonably good diet (lots of vegetables and potatoes and stuff). I'm not overweight, but I do nil exercise because I can't get motivated to do any. For health reasons I'd like to improve on this. Any tips on how I could get more fit/toned, something I could do at home? (I can't afford gym fees)