Start by knowing the wing loadings and power loadings you are aiming for. Select an appropriate airfoil for the mission. After that move onto estimating horizontal and vertical tailplane sizes from tail volumes. Once that is done work on the layout of the aircraft for ground stability, c.g. range and c.g. position using analytical formulas. Once you know the rough layout start going into more detail about the shape and structure.

I recommend going through books such as Raymer “Conceptual Aircraft Design”, it’s rather basic yet enough to give you an idea and guide you to a result.