pseudonym - Using a pen-name: possible legal issues

My question is about publishing under a pen-name. Does it have to be registered as a trademark? If it's not registered as a trademark, can someone sue a writer for using it? Because every name you pick is surely also someone else's name. (Every name I could think of returns thousands of results in Google...) I just basically try to understand how to use a pen name without getting into trouble...Is there maybe a place online where I can get a legal consultation on this topic? Thanks in advance! Jackie