It depends where you live. In California you file a simple form with the County Clerk and then you have it published in a paper for several weeks. Then the name is yours for 5 years.
It is a 2 part process. File the FICTITIOUS BUSINESS NAME STATEMENT, and then publish. State law requires that within 30 days the registrant must publish a statement in a newspaper of general circulation in the county in which the principal place of business is located once a week for four successive weeks. Good info here: http://www.lavote.net/CLERK/Business_Name.cfm Application here: http://www.lavote.net/GENERAL/PDFS/FICTITIOUS_BUSINESS_NAME.pdf Hope that helps.