You've decided on the perfect business name for your start-up, but before you order the letterhead you need to check it isn't someone else's perfect business name, too. Not only that, you have to make sure it's not too similar to another business name, even if it's in another state. It pays to be thorough at this point. At the very least, you could have the cost of changing your name and all your branding. At worst, you could be hit with a nasty expensive lawsuit for violating trademark law.
Start with a basic search online. If a name, or one similar, is in use it is likely to show up here. You need to make sure it is not even similar to any business that offers a product or service like yours. Business names are incorporated by each state in the U.S. If nothing turns up in your basic online search, start with central databases at the Securities and Exchange Commission and Thomasnet. Don't just search on your proposed name, but any variations in spelling or similar names eg: The Big Easy, The Big Ezee; Big Freezeey, etc. You can also check Network Solutions to see if anyone is using, or has reserved, a domain name like yours. Finally, check search the US Patent and Trademark database.
Next, search each Secretary of State's online business registry database. You can find a website addresses and contact information for all states on the Small Business Administration web site. Again, search not just on your chosen name, but on similar combinations. If your search turns up no matches, check with your county clerk's office for a list of fictitious Doing Business As (DBA) names.
If your search finds that there is already a business trading with the same or similar name to yours, you may still be able to use it providing you offer a different type of product or service, or are in different regions. Check with your Secretary of State Corporation Division to make sure you meet any legal requirements.
Once you have verified that your chosen name is available for use then consider registering it even if you are not ready to start trading. Also consider registering your domain name, as well as any domain names you want to route to your website. You will need to register your business name through either your county clerk's office or your state government. Most, but not all, states need you to register a DBA if you are trading under any name other than your real one.
It's a good idea to have several possible business names before starting your search. That way if one is already in use then you won't have to waste time doing another full search. It's understandable to be excited about starting a new business, but doing a thorough investigation at this point really can save you a lot of money and hassle down the line if someone contests your right to use your chosen name.