Once you have chosen a business name, you should make sure that someone else is not already using it in a similar field. The easiest first step is to search an Internet search engine for your proposed name. If it is a common word that brings up a lot of results, add keywords relevant to your business or organizationto see if any similar enterprises have adopted the same name. If you see any other websites doing work similar to yours, you should consider choosing a different name. When you consider whether something is "work similar to yours," take a broad view. This could be anything from a journalism site, a blog, some kind of interactive web service, something tech-y with a heavy online presence, etc. There is little certainty in this area, and you will just have to do your best in making the call. The same goes for all the searching described below -- the task is to weed out names being used by people doing something similar to you, broadly construed.
Next, you should look in TESS, the federal trademark database. Search for the name you're considering as well as close or obvious variations. If anything comes up, check to see if it is "LIVE" or "DEAD." If a mark is canceled, abandoned, or expired ("DEAD"), it probably is safe to use it, but you should do a little extra research to make sure that the company is not still using it in reality. If there are currently registered marks ("LIVE"), look to see with what goods or services the registering company or person is using them. If the company or person is not doing something similar to you, broadly construed, you can probably use the name. However, if the name is famous (i.e., it's a household name like Google or Volkswagen or Prince), you should avoid it. Even if a court would not ultimately conclude that your use of the name constitutes trademark dilution, large companies with famous marks can be very aggressive in policing the use of their trademarks, and they are likely to have highly trained lawyers to make your life difficult. In any case, if you choose to use a name that someone else has registered in connection with a different kind of business, be sure not to use a similar logo, design, or color scheme.
Even after checking the federal database, you're still not necessarily in the clear. Each state and the District of Columbia has its own trademark registry. You can find out where your state's registry is at FindLaw's State Trademark Information page. States have their own trademark laws, which are generally similar to federal law and can be just as powerful if you are sued (or choose to sue someone else). You should perform a search for your business name in your own state and any other state in which you particularly expect to do business or become involved (beyond simply disseminating information to residents of a state via the Internet). To be completely thorough, you would have to search each state's database individually, which would be tedious and time-consuming. You can decide whether this is worthwhile. You can also pay an attorney or one of several Internet-based services to conduct a trademark search for you. FindLaw has more information on hiring a professional firm to conduct a trademark search.
Even if you find nothing through all of these searches, it is still possible that someone somewhere is already using the name you intend to use. This is true because federal and some state trademark laws protect unregistered trademarks, and there is no comprehensive list of all unregistered trademarks in use. The searches described above are the best you can do, and if they turn up negative, the chances that you will run into trouble are greatly reduced.