Ok, texture your planet. This tutorial isn't designed to talk about texturing planets (a whole book can be written on it). Rather, we're just talking about how to render planets/moons for another use.
So do whatever you want that you think will look cool!
Turn on the production frame. This will show you what parts of the scene will actually get rendered.
When you look through the camera that will generate your final render, you want the production frame to just be on the edge of the planet. You can adjust this by changing the Y position of the shot camera. You can see this in the provided image. This will produce a square image that will be just of your planet/moon.
If the moon/planet is not fitting just within the production frame. Move your camera by adjusting the Y position of it until it does fit.