How big to you want to get? For starters, I wouldn't worry. Small amounts of income, hobby breeders in essence, generally don't claim any income, collect taxes, or anything of the like. In Canada, if one makes over 30K, they must collect and pay GST (tax) to the government. Provincial and State taxes probably have a similiar ruling, albeit my province has none, so it's an individual thing.
My best recommendation, is if you decide to make this a significant ordeal, you'll need an accountant. The best thing about this, is they'll also be able to answer all these questions specific to your situation and location. If you're just selling one litter a year, I'd just keep it all under wraps.
Ian
ps, look into this more, as I'm no pro when it comes to finances