Use on when you are talking about a surface.
Ex: I'm walking on [the surface of] the road.
Ex: I am on sleeping on [the surface of] the beach.
Ex: I am surfing on [the surface of the water].
Use in when you are talking about inside of something.
Ex: I am walking in the water.
Ex: I am walking in the sand.
Ex: My shirt is in the closet and it's hanging on [the surface of] the wire hanger.
Transportation is a bit different:
Ex: Get in[side] a bus, plane, a small boat.
Ex: Get on[board] a bus, a plane, a ship, a bike.
Cars and taxis:
Ex: Get in the car, the taxis.
Never, *get on the car, the taxi. That means get on top of the vehicle.
Does that help?
Student or Learner