Choose what business type you are going to set up and register the business with your state. In my case, it's Illinois, and you can register it here: https://www.revenue.state.il.us/app/ibri/
If you've chosen to open your business as a Sole Proprietor, but you want to do business under a name besides your own, register an "Assumed Name" or "Doing Business As" name with your County Clerk (at least, that's how it works in Illinois).
Step 3 (optional for Sole Proprietors):
Register with the IRS for an EIN or Employer Identification Number. If you are going to employ people, this is required, if not, it's optional, but I've read that sometimes other business that you interact with may want it and banks sometimes need it if you want to open a business account. It's free, and if you register for it online you'll get it instantly. Link: https://sa.www4.irs.gov/sa_vign/newFormSS4.do
I completed steps 1 and 3 on the same day and got confirmation back from the State of Illinois and got my EIN already. The state also sent forms so I could pay my sales taxes online. So, as of July 1st I've started a business.
What remains to be done:
Step 2, and then I also have to find out what needs to be done so I can NOT pay sales tax on purchases that I intend to resell (like frames, matting, etc).
Other usefull info for Photography Businesses:
- Is photography retail or a service? Do photography sales incur retail tax?: http://www.ilga.gov/commission/jcar/admincode/086/086001300S20000R.html
- Awesome e-book for anyone thinking of getting into any kind of photography business (includes how to price, etc): http://www.danheller.com/bizfaq.html