Of course fasting works. I'm not an expert but I'd estimate you could expect to loose up to 10 pounds of weight a week by "pure" fasting. That is a water only diet. The average body in order to support basic life functions needs to burn approximately 1200 calories daily.
Fasting is extremely difficult, and requires great discipline over the course of one week. Don't substitute eating for consuming wheat or sugar juices (both are popular fasting methods but only result in substituting one form of calories for another). Be sure to keep hydrated by drinking plenty of water.
While fasting for a period of 1 week may not be life-threatening, most dieters will not be able to tolerate the hunger pangs for so long. The issue with fasting is that your body's metabolism
will also slow down so that fewer calories are burned per day making future weight loss
through other means more difficult (I've heard this claim but am doubtful of its validity). Also in place of burning fat your body will eventually begin to burn proteins
(muscle tissue) so that you'd take on an unnaturally unappealing shape (think Anorexia nervosa
). Don't believe the hype that fasting is medically bad for you over short periods
of time. It may or may not be, some medical researchers will even claim that fasting promotes health but the research has not been validated.
I'm worried about you developing anorexia, but do believe that improving your self esteem is important so I'm not going to criticize your desire to loose weight. If anything talk to your parents and hopefully a medical professional about your concerns. There are other probably better solutions for example jogging regularly. Come to think of it, wouldn't hurt much to get into an exercise routine while fasting. Exercising while fasting will help burn calories faster.
I'm sincerely thankful you decided to ask for help even if it is from the yahoo! community rather than to simply go ahead and fast with ignorance.