You can sell as a hobbiest without having a business. The one thing you'd need for that is liability insurance. Liability insurance is probably one of the most important things you'll need to get before you sell a single bar of soap.
When I registered my business I filled out an application online with my state. If you look up your state name, and then "business license" I'm sure you'll be able to find all the info you need and the steps you need to take. My state does charge sales tax, and so around November they sent me forms in the mail to fill out and pay my sales tax.
This article by Marla Bosworth is by far the best and most detailed article I've read about starting a soap business. I highly recommend it before you get much further!
http://blog.worldlabel.com/2010/how-to- ... iness.html