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I need some fitness advice.? - pilates fitness equipment

I am 13 years old, and I'm 5'7 "and 1 / 2. I weigh around 160-180 pounds. Not that I eat junk food, it's just that I do not exercise it. I want to lose 20-30 pounds for my graduating from 8th grade next year. The date of graduation is scheduled for 28 May 2010. I would also like some turns. I need your abdomen, arms and legs. I heard from my friends Pilates and yoga that are very good tone and heart. I do not have the equipment. It's very expensive to get a subscription to the gym. I've also heard that it was also good aerobic. Has anyone advice what I do is what would happen - the most effective? What has me the most?
Thank you. :)