The biggest idea to give up is that there is an absolute state of rest. There is no universal stationary or rest state. You can only measure velocity relative to something else. By convention, we measure velocity for many things relative to the surface of the earth because that’s where we exist. We require constant energy to walk forward because we have to push against gravity each step. Cars and trains require constant energy to move forward because of things like friction/drag. In a vacuum (ie space), there is nothing to stop an object from moving. So once it’s set in motion, it will continue in that state until something changes it. But remember again that there is no standard motionless point in the universe. Everything is technically in motion and at rest at the same time depending on how you measure it. We measure a car’s speed relative to the surface of the earth but the surface of the earth is spinning very quickly. The earth itself is moving very quickly around the sun. And the sun is moving very quickly around the center of the galaxy.