The best thing for your body is NOT losing ten pounds in a week. Ever. Safe weight loss -- when you actually have weight to lose, as okayed or advised by your doctor, not just because you want to be thinner than you are to meet some beauty ideal or another -- is only about 2 pounds a week.
We do NOT advise dieting -- especially fad diets like Atkins KNOWN to be unsafe, see: http://www.scarleteen.com/politics/gender.html -- at Scarleteen, period, especially for folks who are still growing and stand to really screw up their bodies lifelong by doing so. As well, this is not a weight loss site.
If you want your body to be in the best shape it can be, the formula for that it reaally simple: get at least 20 minutes of good activity and exercise a day (there is no single exercise that makes everyone "lose weight the fastest." People should just choose exercise based on interest, suitability and what they feel challenges them the most). Ditch processed and junk foods and heavy fats and sugars. Eat healthy foods in a sane balanced diet of fruits and vegetables, lean or nonmeat proteins, and complex carbohydrates. STay hydreated with plenty of water and get enough rest.
Really, those things -- unless you have other medical conditions for which you're advised otherwsie -- are all you need to be doinng to be in the shape you're meant to be, or to get into a healthy one that'll benefit your body for life.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson