I'm not familiar with how much a stone is, but the best way for a young lady of your age would be to make sure that you eat a good balanced diet, cut out sweets and sodas and exercise every day. good luck!
You have a Body Mass Index of 28, which would be considered overweight for an adult. Because teens are growing rapidly, BMI is not always the best indicator of a healthy build. If your waist is more than 1/2 your height (79 cm), you probably should try to get down to that 79 cm waist. teenDailyStrength offers some advice on losing waist.
You need to do some exercise, even if it is as simple as walking more. Find something you like (dancing?) so that you can stick with it. Cut back a little on how many calories you eat. Don't do anything drastic, or you'll most likely not be able to stick with it. Eat smaller portions or give up one soda or dessert a day. Good luck!