I don't think 12 is too young to start, though it might be too young to make a lot of money at it. You don't need a Nikon to start out with. Any digital camera these days is pretty good. Plus there are advantages in digital cameras, you can take as many pictures as you want because they cost almost nothing. When I first got into photography I had to look through the viewfinder and think to myself "Is this worth 25 cents?" Because that's what it cost to take a picture on film.
You need to read a little about photography and learn some things. The relationship between f-stops, shutter speeds and focus. How to use light to 'model' shapes. Stuff like that. Photography is like half science and half art. That's one of the things I like about it.
You can download a program called GIMP (GNU Image Manipulation Program) that you can use to edit pictures. And you want to learn to do that. Any picture you take can benefit from at least adjusting brightness and contrast and 'levels'. Later, when you earn some money, you can buy Photoshop, but GIMP is almost as good and it's free. You can download it at http://gimp.org.
I've heard there are places online where you can edit pictures but I don't think that would work very well because the picture has to be sent back and forth while you're doing it. GIMP would be better. Then get an account on FLICKR and post your pics there, and let us know and we'll critique them for you.
Good luck!