If you're going to be a sole proprietorship, it's just as easy to form an LLC or S-corp. Read this thread:
http://forums.monstersmallbusiness.com/ind...p?showtopic=681BASIC STEPS (may differ slightly from state to state)
Wizard has a very important caveat in his post - check with your CITY first if you are going to be working out of your house. I had to file for a special permit for home occupation and get approval through both the planning and zoning boards before opening. No yard signs allowed. No foot traffic. I also had to have a water meter installed.

Don't ask. If you live outside of a CITY, check with your TOWN to see if they have any home occupation requirements.
If you go the sole proprietor route, go to the COUNTY office to apply for a DBA.
If you go the LLC or S-Corp route, probably best to consult with an accountant and/or attorney if you're not familiar with tax entities. I found alot of useful information at bizfilings.com and actually used them to form my LLC. If you'd rather not pay for an accountant or attorney at first, call bizfilings and they'll answer your questions about incorporating for free (just tell them you're interested in using their services and have some questions - worked for me and I went with them).
And then the wonderful resale certificate. Check with the STATE to see if you need to collect sales tax and/or how to go about applying for the certificate. (If you're going to get an EIN - see below - get that first before filing for this certificate and use that number instead of your SSN).
The IRS does not require an EIN if you are going to file your business income on your personal tax return. However, 99% of your suppliers and the bank (and anyone else that you will come in contact with re: your business) will ask for it. It's free, so why not get one. As Krazy said, you can call the IRS and get it in 10 minutes.
That pretty much covers the basics. Good luck!