You'll have to lose weight all over, unfortunately. There is no way to drop fat from a given area (Oh how I wish there were - I'd just lose weight around my abs

)
These guys are right about diet. I'll try to address the exercise end. Pick a sport and play it two or three times a week - play hard. Barring that, run, bike, swim or skate instead. Once you have a decent level of cardio fitness, incorporate intervals -- slow/fast bursts of energy. An example: sprint for 30 seconds, then jog for a minute, then sprint for 30 seconds, etc.
You'll want to target every muscle group when you lift, but since you asked specifically about the legs, the building block is the squat. For other good exercises, go to
www.exrx.net and look under the quadriceps and hamstrings sections of the exercise guide.
Rock out.

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