You need to determine whether another business is already using a trademark that's identical or similar to the one you want to use to ensure that you won't violate someone else's trademark rights.
If you are sued by a trademark owner for using its trademark, at the least you can be forced to stop using the trademark. Depending on how long and extensively you've used the business or product name, it could be costly -- you could have to change products, brochures, letterhead, business cards, signs, advertisements, and your website.
And, if you use a federally registered trademark improperly, a court will assume you knew it was federally registered, even if you did not. This means that you will be cast in the role of a "willful infringer." Willful infringers can be held liable for large damages and payment of the registered owner's attorney fees.
For help with choosing a unique mark that competitors can't copy, get Trademark: Legal Care for Your Business & Product Name, by Stephen Elias (Nolo). For help conducting a trademark search, see Nolo's article How to Do a Trademark Search.