Having gone through trademark issues, I can help you with this.
Regardless if you have a DBA (Doing Business As, or fictitious name) registered locally, it does not necessarily mean that you own it. Although with costly court hearings, you can plead your case if you choose to do so.
The best way for any new business that you plan on starting is checking out patent office site, www.uspto.gov
and see if your name is trademarked. Now, one thing to note is that if your name sounds similar to a name that has already been trademarked you can get a cease and desist letter from the company to stop using the name. But if your trademark is pending, you amy be able to fight them on it, at least for a little bit to see the outcome. If your trademark goes through, then you don't have anything to worry about. I have heard other small business owners tell me they trademarked their names themselves, however, with my company I wanted to make sure that all of the proper marks were registered as well as ensure that the proper research is done. This is a bit more expensive than doing it yourself, but it can mean the difference between getting the trademark and not getting it.
People have also mentioned the notion of first use. This can apply but again, it can be costly if another company says you are infringing on their trademark. Costly both in the downtime you have to take because you can't market your company and the time it takes to either fight them or change your company name which means changing your packaging, marketing materials and everything else.
So make sure that you research this first and if you have a great name, begin the trademark process. It can cost a few thousand dollars depending on how many places you want the mark to apply, such as food and beverage, clothing, and other areas of business.
Hope this helps. Good luck with your company.