How long shall I exercise on these in order to lose weight?

Well I'm 12 years old and I weigh 6st 5lbs the doctor said I have this condition and I need to lose weight fast because my body's not used to it and I need to get rid of 6lbs So are there any exercises i can do to get rid of it the only exercise stuff I have are the : wii fit plus , trampoline, treadmill (that goes up to speed 8.8 mph ) and just dance 1&2. So how long shall I go on these for ?
 
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