It's a business - so you should be required to register it, same as you'd register any other business. I don't know of any US states, at least, that don't have some level of business registration requirement.
Check with your county office and they'll tell you what you need to do to register your business in the area where you are, as well as what taxes you may need to pay/file/be aware of. They may also tell you what state filings you need to make.
If you're in the US, look for books by Nolo Press (www.nolo.com
). They detail starting up small businesses fairly well.