How to get a nice body quick?

i am a 15 year old girl im 5'7 and weigh about 135...im not like really fat i just have chub on my tummy and a little on my legs and arms...nobody can really tell though i do a good job hiding it cause its not bad but bikini season is really soon and i just want to loose my fat so i can wear a bikini comfortably how much do you think i need to loose to get that flat firm stomach i want?