Most people know they need a business license, but you may not know where to get one. In some places, you get that from the city. In other places it might be the county, the parish, the district, etc.
In states that collect sales tax, you need a permit that allows you to buy hardware and software and resell it, collecting that sales tax and paying it on to the state government.
All of this starts with the decision about the form your business will take (S-Corp, C-Corp, LLC, etc.). But after that, there are layers and layers of government that want to regulate what you do. You may need permits to work in someone's home, or to install cabling, for example.
I posted this video on the subject today:
Of course you're going to have a tax ID number for your business. This might be your personal Social Security Number, an employer identification number, or a state-issued tax ID. The form of your company affects the way you file taxes, sign agreements, and run you business.
Licenses and Permits can be overwhelming. You need to do the due diligence to find out which permits you need, how you should be signing contracts, and which taxes you need to pay.
Here are a few links to get you started - at least in the U.S.
In California, we have more layers of government and taxation than you can imagine. Almost no one here knows all the regulations they're supposed to follow! But we have a "golden" web site that's supposed to help you get started. It is called CalGold: http://www.calgold.ca.gov/.
If you know of similar sites in other states or countries, feel free to add them to the comments.
Remember: This is manageable! Lots of people have done it and you can too. So don't be overwhelmed.
Editor's note: Updated link on Jan. 10, 2019.