background:
when i was 14, i was 240 lbs and i was just 5'6 then.
then i started on a diet then went down to 190 after about 3 months.
when i turned 19, i was already 5'11, and i was still 190 lbs..
then i started going to the gym, and i dropped down to 170 lbs (my ideal weight)
i stopped going to the gym, and i went back up to 180 lbs.
my advice is, work your ass off.
7 day diet: first 3 days, the amount of food intake should be "just right", next 4 days, eat about 1/2 - 3/4 of the "just right" amount. it will do something to your metabolism and you'll start losing weight faster.
start running, jogging, hopping, jumping, or any aerobic workout.
then 1hour after the workout, eat your big meal for the day. according to my gym instructor, your metabolism is still up there at that time, so you won't really gain so much weight..