You don't start by selecting motor and prop. You start with the type of airframe and what flight characteristics you want it to meet. Then you look at the wing loading to determine how much weight you can have in the airframe. Next you'd look at what speed and torque is needed for the style of flight the airframe supports. Another ( but not the only thing ) is how tall does the airframe fit so you know what the maximum size the prop can be.
I always recommend when you don't know that information is to instead start with a known airframe that already has recommended components selected. Aeronautics isn't rocket science but it isn't simple either and there are a lot of complex ideas that must be known first before starting on a new project.