With your strenuous and stressful lifestyle, one of the worst things you can do is allow yourself to become malnourished and underweight. You need the calories and nutrients (and a proper amount of body fat) to fuel your body during all the soccer games, pop quizzes, beauty pageants, etc. that you do.
Eating healthy is a good idea. Follow the food guide pyramid for suggestions on serving sizes. Limit fats and sugars, and load up on lean proteins, whole grains, low fat dairy products (especially important for teen girls), and colorful fruits and veggies.
If you want to get in better shape, fuel your body well with the proper amount of calories (information is available on how many calories you need daily based on your height and activity level)and nutrients, drink lots of water, and get plenty of exercise, including cardiovascular and strength training, and don't forget to get enough quality sleep (as a teenager, you may need more than 8 hours). Don't think about losing weight...think about making your body as healthy as possible. You may see positive changes in your shape and your health even if you don't see a difference in the scale.
Not only is being excessively thin dangerous for you, but you will find that you don't have the energy and stamina for the things you love, and you may even meet bias from pageant judges who are unhappy seeing a gaunt, sickly-looking teen.
As far as BMI goes, for a healthy teen, 20 to 25 is normal. Nineteen is considered underweight and is risky. I don't know what your BMI currently is, but if it's already at 20, there is no need for you to diet or cut calories. You are already as thin as one should be without endangering her health.
Women require a base amount of 10% ESSENTIAL body fat. We naturally have more body fat stores than men, and this is OK. A very lean female athlete may have a body fat percentage between 10% and 20%, and up to 35% for a female is considered healthy. You have a lot of room to play around with body fat, but you will not reduce your body fat simply by restricting calories.
If you want to bring your body fat percentage down, a few things you can do is focusing on exercise to build muscle strength and burn fat (drastic dieting often has the effect of causing loss of lean muscle tissue so that while a dieter may be losing weight, he or she might actually be "fatter" than he or she was before the diet) and eating a very healthy diet. Information is available about ways to safely reduce body fat.
Sweetie, losing weight and/or abusing your body will not make your life happier or more satisfying or make you more successful. We all have this idea that if we change what we deem physically imperfect about ourselves, our lives will magically be better. And our weight and the amount of food we consume is something that we can control when everything else in our life seems chaotic and it feels so good to have at least one thing that we, and only we, can control, but there are real risks to becoming too thin. Being underweight affects your fertility, puts you at greater risk of becoming very sick (or even dying) from a simple illness, and can put stress on your heart, among other things.
I hope you'll consider the things I've written...you're far too vibrant and articulate to be doing this to yourself.
Best wishes.
