If you have reported your height and weight accurately, your
BMI is 20.7, which is perfect. You do not need to lose
weight. If you did need to lose
weight, it would have to be under parental supervision. The type of normal play that nine year-olds do should provide you with enough exercise. In any case, if you are truly nine years old, you should be talking to your parents about these issues and not asking advice from random strangers on the internet.