You don't have to register or file any paperwork with the federal government to form a sole proprietorship. If you go into business without setting up another business structure and if you're the sole owner, then you're automatically considered a sole proprietor.
However, some states and counties may require you to obtain business licenses and/or permits before you can lawfully operate, Also, if you want your business to have a name that's different from your own legal name, then most states will require you to file for a DBA.
Learn more about sole proprietorships and how they compare to more formal business structures.