If you are aiming at losing weight, it is important to calculate your current BMI and the target BMI. The target BMI should be in a healthy range. The key is to follow a healthy diet plan with a balanced amount of physical activity. A healthy diet should consist of adequate calories according to ones height, weight and physical activity; with adequate amounts of carbohydrates, slightly increased proteins, little fat and plenty of fruits and vegetables to supply vitamins and minerals. Start with a healthy breakfast, moderate lunch and light dinner. Also keep a track of calorie intake and your work out. As far as the physical exercises are concerned best results can be obtained with rotating between various activities like gymming, running/ jogging, yoga, pilates, aerobics, biking, swimming etc. Though one tends to lose weight faster with crash diets, these are not typical advised since these are usually followed by weight gain which is difficult to lose. Weight loss needs to be gradual to be healthy. With gradual consistent efforts, you should be able to lose weight in a healthy way.
Hope this helps.
Ten stone is in the high side of the average BMI group (around 22 I think.) 126 will put you a little lower and is more realistic. Less than that and you'll actually end up underweight (going by BMI alone, everyone is a little different.)
Keep track of your diet and make sure that you are getting all the nutrients your body needs, and reduce your calorie intake a little at a time-- no junk food, no sodas, no empty calories anywhere. If it doesn't have a high nutritional value, try to give it a pass. In addition, get your heart rate up to put your body in a calorie burning mode and work on building muscle through strength and resistance training. You might end up weighing about the same but it will all be lean athletic weight (which looks great and feels good and is the healthiest way to go.)
I think it is great you want to change to a healthy lifestyle, although i would like to point out the dangers of having to change to 'be skinny like everyone else'. I only speak from experience and know far too well that eating disorders are very tough to overcome.
Saying that, you can start by looking at what you actually eat. Are you eating convenice food, snacks, white breads and pasta etc. Also look at your portion sizes. I found that simply making sure 50% of what was on my plate was veg (not carbohydrates) and cutting down on my portion sizes worked really well, and is very cheap.
You can also be more active, taking a run around your street, or walking to school are both activities you can do at home which would help loose weight. Try joining an after school club like netball or rounders to get more active.
You are in the normal weight range for your age, to whilst living healthy is a good thing, dont let it rule your life! The general rule to to expend more engery than you consume.