In the U.S., you should read about the LLC, S-Corp, and C-Corp. They have their tradeoffs depending on what your goals are.

One other thing I'll mention is that it's really important to have 2 or 3 founders. Being a sole founder in a startup is very difficult. Not only is there a lot of work but it's often an emotional roller coaster so sharing the experience with someone is immensely helpful. Not to mention having someone to brainstorm with when making the big decisions.

1 - Figure out a business name and Register it at your local register-recorder/county clerk office. You must make sure the name is not already taken. This is called DBA Fictitious Name

2 - You may be require to get a City License. You may need other licenses and certifications as well to be able to operate your business depending on what type of business it is. So do a deep research on this and make sure you have yourself covered.

3 - If you are reselling tangible goods, for most states I believe you will need to carry a resellers permit which you can obtain from the state board of equalization.

4 - This next advice is optional as you may want to consider becoming a corporation or LLC (Limited Liabitlity Corporation). This is to protect yourself by separating your personal assets from the business. This is especially recommended when you have partners.

5 - Get an accounting software to manage your books. Quickbooks is the most common software used, but of course you are not limited to choices out there based on your needs. For basic, Quickbooks Rule!

6 - Get Insurance. There are many types of insurances out there, get what is needed, nothing more. Liablity is something that you may need to protect yourself from lawsuits. Theft for inventory. Workers Compensation if you have employees. And much more. Talk to a business insurance specialist that you trust, or talk to several to compare notes and prices.

7 - You will most likely want a Website and Marketing done to it. For most business, internet is a platform you can't afford not being on. Here is some of the options available.