Usually you would start with the propeller and work back to the motor.
A given propeller will have a characteristic performance, usually a curve or a chart. For various speeds of rotation it will generate a particular amount of thrust and require a certain input power from the motor.
First you will choose a suitable propeller for your application, one which will produce the required thrust at an appropriate speed. From the performance map you will read off the necessary mechanical power to drive the propeller.
Given this information you will find or choose a motor that can produce the required power at the necessary speed.
If instead, you have the motor first, then you would have to select a propeller to match the motor. In this case you would find the optimal operating speed for the motor where it produces the maximum power output without overheating. Then you would choose a propeller that consumes the right amount of power at the speed of the motor. In this case you will get whatever thrust the propeller produces.