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Sensible and steady weightloss involves workout + diet managment + lifestyle modification.
1. Visit a dietician and have a diet planned , according to your age & sex. Dont try crash dieting, you are still growing , so it will cause growth & hormonal
2. Start a work out regime, or even better a sports acitivity like tennis or swimming, it works really well and is fun too.
3. Modifying lifestyle like sleeping at right time & waking up early, helps a lot. Avoiding junk food and eating more of home made, veggies, fruits , milk & milk products is important.
4. Also visit a doctor for an examinaiton & routine tests to rule out any other medical condition. You havnt mentioned wheater you r M or F. If you are a female, please make sure your menses are regular.