Requirements vary from state to state. I have spent the morning looking through the site linked in a reply on this thread:
https://www.etsy.com/teams/5002/etsy-success/discuss/14782101/page/25from: "Matthew and Sara from ananemone says:
egustaf, this blog is where I learned pretty much everything about the business/law end of things: www.gotogreatpanes.com/blog/sales-tax-business-registration/
It's fairly thorough (she has other topics, on the bottom left hand side of the page).
My advice is to do all of that stuff now. It is harder to do afterward than before.
Posted at 12:57 pm Aug 8, 2014 EDT"
I would also suggest looking up your local SBA office. They offer free counseling to people wanting to open a small business.
You definitely don't want to get into trouble with the IRS!