i am 12 and over weight

Well your body is still growing up but if you're really in need of fast quick results start looking into water fasting or juice fasting or / and daily work outs that fit your needs and a calorie intake diet

you just need to cut out sweets cakes etc.and.use a salad plate instead of a dinner plate and do exercise regularly but dont go on any diets or fasts at your age just dont worry and take your time and dont be influenced by your peers to be stick thin!