Welcome to HCM,
Look you are only 15 years of age this adolescent over weight,your weight depends on your height once you know your height,we can calculate BMI.
If your height is 150cms you will be marginally over weight at this age you need good nutrition and calories for growth.
First consult a local physician to rule out any organic pathology or hormone disorder by examination and investigations like CBC,FBS,Creatinine
and x ray
chest and ultrasonography
It is also important to consider genetics.
If required other special test may be required.
If every thing is fine,please take up some outdoor sports and cycling,jogging and also taking care of diet avoiding to much sugar,fats,fast food chocolates,cakes,pastry,and ice creams.
I am sure with you can certainly become fit and healthy.