i'm 21, 5'11 and like, 9 stone 10 or something. i've put on a bit over christmas but i'm allowed to hahahaha yolo
best place to start would be to clean up your diet, get your mum or dad to help you like in terms of the kind of food they're buying and cooking for you and limit things like fizzy sugary or salty food/drink
start out walking loads, including hills, then maybe move on to fitness DVDs/youtube videos/boxercise classes/zumba etc, or you could try running intervals. there's an app called couch to 5k which directs you through walking and running intervals 3x a week for 9 weeks til you can run 5k, even if you don't go through til the end it's still a good place to start and get your confidence up!
i mean, at the end of the day you're 13 and shouldn't get bogged down in calorie counting and BMi calculators, but if you're not happy in yourself then there's only one person who can change that! good luck xxxx