Some great advice has already been given, however, I'd add;
- You have to spend money to make money. If you want to do well, I'd suggest signing up for a small business (entrepreneur) course at a local college. There is not usually an age limit on these courses. Maybe you can find a night course in your area. You can learn a lot about the fine points of setting up your own business, how to handle finances, creating a professional business plan model, and all the things that go along with it.
- If you just want to breed them and sell them tax-free at local shows or through classifieds, you won't need to register as a business. Check your local taxation laws, just to be sure you (or your parents, if you are a minor at the time) can't get in trouble for undeclared income.
- Be sure to establish a business policy in writing that is available to your potential clients - something you can realistically live up to.
- Plan to deal with difficult customers and how you will handle those situations. If your people-skills are not what they could be, take a weekend or night course on Mediation. It will pay itself off big-time eventually.
- Don't forget the internet; when it comes time to sell, get yourself a good website or, at the very least, set up some ads and post in forums. Again, you have to spend money to make money.
Good luck.