I can honestly take a look at myself in the mirror and tell you without any fear of contradiction that regardless of the potential I may have had when I was thirteen my competitive racing days are now over. However, judging by the looks of you, I am sure your best days are still ahead of you. For you, training to be an Olympic mountain biker should be just beginning. Obviously, the best thing you can do is ride, ride, ride. I would also advise you to take care of your body by giving it plenty of good food and water.
Rest is also important.
Those are the basic steps and the easiest to follow. Also, look for area competitions where you can enter races and begin to test your skills against others your age and with similar experience. There are also camps you can attend, just like with other sports, where pros can teach you some of the tricks it took them years to learn. And if at first you don't have as much success as you thought you would, try harder. The true champions never give up.