you don't need to go anywhere or spend money to get fit, try pressups, situps, body squats, pull ups on a door frame, star jumps etc. use tin cans or bottles of water as weights for arm exercises. housework and hovering are all good exercises, running up and down stairs or steps is great for leg work and cardio stuff. jog on the spot whilst getting dressed, dance to your favourite music for a while each day, skip/jump rope, help with the gardening, teach yourself to juggle using rolled up socks or try hand stands/ walking on your hands (even if you don't succeed you get exercise trying!) put things you use a lot on the top of the wardrobe or top shelf so you have to stretch to reach them, keep shoes under the bed, pushed right under so you have to bend down and stretch to reach them after all every little helps! the main thing is to keep moving, don't sit and watch TV or play on the computer for hours on end, get up and move around. at meal times watch your portion size as well, try making the effort to leave a little on your plate at the end of the meal, that way you will have eaten less than if you ate the lot. remove visible fat from meat, scrape butter/margarine on your bread rather than load it on, softened butter spreads easier than hard butter so you use less, have fruit with your cerial for breakfast rather than toast, drink a glass of water 10 minutes before your meals, you will not feel so hungry and the water will make less space for food in your tummy so you will feel fuller sooner meaning you will automatically eat less. when you feel hungry between meals drink a glass of water and wait fifteen minutes before reaching for a snack, often you will find you don't need the snack as the water has satisfied the craving, if not then diluted sugar free cordials make great ice lollies for those times when you crave a sweet snack. sugar free jello also works. at 15 you probably still like sweets, so remember that jelly sweets like haribos are less calorific than chocolate bars and baked crisps/chips are better than fried ones.
just remember at 15 you still need plenty vitamins, minerals etc as your body is still preparing for adulthood so do not go mad cutting out loads of things, set yourself a target of losing 4 - 6 pounds per month, at your age it is much better to watch the amount of sugars and fats you consume and use exercise to bring your weight in line. remember also that fitness is more important than weight as muscle weighs more than fat does.