Great to have you on the forum!! I would stay away from machines at all times. Scientifically speaking, machine training is very detrimental to your neuromuscular development and at your age, your nervous system function can be hindered by influences you receive now. My suggestion would be to find a qualified professional to consult with a few times so then you can learn the proper techniques that are essential to exercise. If the professional tries to toss you on a machine of any form, (treadmills included)seek another trainer for help. At your age you should be learning movement skills and body weight exercises with proper repetition because muscular endurance is easily developed at your age. I hope this has helped put some things into perspective for you.
Will Haskell
Athletic Development Specialist
www.tsanorthwest.com