Get in a bus (go through the door) or get on (walk on) a bus.
Here's a trick to remember: if you bend down to enter a vehicle, like a taxi or a car, use "get in"; if you stand to enter a vehicle, like a plane, a ship, a bus, use "get on".
It's different for boats. Use get on if you walk on and get in if you bend down.
All the best.