At this age, I'd advice you to run, stretch, run some more, and start doing some shadow boxing.
Running is needed for the endurance and strong legs. Stretch you won't injure yourself, and shadow boxing helps your hand-and-feet co-ordination.
I also advice you to do some light weights to get a fundamental how to lift. Ask Dereck or Cord about this.
I got two fists.. Don't make me use my head as well!