I raced as a very competitive Jr as well as a few others I know on these forums in the late 80s and early 90s when Jr racing was huge. Fields of 100 or more Jrs was not uncommon around the U.S. I hated to train, but loved to race. Now I like to do both.
First and foremost, make your rides fun. That will keep you going. If you are not having fun, there is no reason to do this sport. Ride with people you like, ride places you like.
Second, I would recommend to train by time not miles. It will make training more fun and you won't be worried about getting miles in and making your ride too long or too short to be at that mileage you set for yourself. At the age of 13, I would say 4-6 hours a week on the bike is good if your rides have some structure. Structure is hard when you are young. Most younger atheletes like to do what they are good at (don't worry, adults are the same way..LOL). If you suck at hills, do lots of hills. If you can't sprint, work on sprinting. It doesn't have to be scientific.
Try to ride with someone older and more experienced that may be able to set some goals for you and help you get some direction.
Good luck and keep us posted.