Xaccers is 'spot on' about exercise being important, as the bodies natural reaction to reducing calorie intake is to lower metabolic rate to compensate, which stops you losing weight.
It isn't easy, I guess 'at your age', but even a brisk walk each day is better than nothing - provided you don't try to do too much, too soon.
The cabbage 'diet' is very poor, Xaccers again hits on the key thing, which is balance, so you get all the nutrition you need, but, overall, reduce the calories. Zinglebarb posted a link to a site which allows you to input what you eat & what exercise you've done & works out your overall net energy intake, if I can find it, I'll post back.
here you go, link in first post of this thread
http://www.cableforum.co.uk/board/sh...ad.php?t=50040