eliptical ski-ing machines in the gym are menat to be fab for this. Also swimming.
You need to exercise for at least 20 minutes to warm your muscles up and then any additional exercies time after this will burn off calories. So if you do a 45min workout you will burn off 25mins of calories.
All the best to you. xxx
You might be asked to be accompanied by an adult and certainly vouched for, but why not see if there are any local fitness classes you can attend after school? That would probably be a lot cheaper for you.
As mrscheeks says, you need to warm up for 20 mins first so go for a jog, and if you cant get to a gym or afford one (they are a bit steep on price) then to concentrate on your thighs alone, take a few trips up and down the stairs each day.
To lose weight off them kind of jog up and down say twenty times per day.
or:
To build a bit of muscle on them go slowly two stairs per stride say ten times per day.
In six weeks Beyonce will be jealous of ya !