In the internal view (F1 to switch between external and internal views, and F8 to switch between internal views) there are a list of autopilot functions along the bottom. You can click them to activate them, or use the keyboard shortcuts. L for horizon level, A on some crafts to hold altitude, Numpad 5 to kill rotation, [ to activate Prograde, and ] for Retrograde. There is also heading and speed autopilots for some spacecraft. I would recommend looking in the Orbiter directory under the folder “Doc’ for documentation