It takes more than just an interest in history to become a historian, which means that even though your Uncle Leo might get riled up about the hostility of the North during the Civil War (and shows up to family dinners wearing a dusty Confederate gray uniform), he's not a historian.
Actually being a historian is something very few people can claim. To become an authority in all things historical you must spend your fair share of time in school. A bachelor's degree is the least of your troubles; most historians have a master's in some kind of historical specialty, like revolutionary America or Elizabethan England (source).
Certificate programs focusing on teaching you specific skills that you'll need on the job come in handy too (source). These are offered by all those fancy-shmancy historical societies. We imagine you'll take the exam at a mahogany desk while a pipe-smoking old man stares at you over his mustache.