how do you get below deck on a ship? like a ship made in the 1800s?

hatch and ladder

Crew went below via the focle hatch. Usually a fairly steep stair way inside.

Capt via a door on the main deck to the aft castle or poop deck. Officers also used a similar door.

Almost always a ladder arrangement. In a very few cases there might be stairs to a lower deck, but stairs take up valuable space in tight quarters.

Down the hatch!

come back when you sober up..