go to your county clerks office- they will have an application- and fees- (they vary state to state-county to county- city to city)
first they will (do a search) on the name you want to use to make sure someone else does not already have it-
if no one does they will allow you to have it-
it will be published as an assumed name-
then you have to file for a tax id---another process- the clerk should be able to tell you how- may have those forms too- if not should tell you how to get them.