Welcome to the forums.
Here are some articles that may help.
https://www.flitetest.com/articles/Design_Parameters_for_Scratch_Built_Airframes
https://www.flitetest.com/articles/Basic_information_for_Trainer_design
https://rcplanes.online/design.htm
If this is your first plane, I would recommend starting with a foamboard plane, from a known design. Pick one of the many FT plans and build it. We will be able to give you far more help if you are using a known plane. The tiny trainer would be a great choice.
After you learn to fly, make the plane of your dreams. Go for the 3D printed plane. 3D printed planes tend to be heavy, which means they must fly fast, not a good trait for a beginner. A fast plane means less time to react before it hits the ground. 3D printed planes tend to be fragile, they will break far easier than a foam board plane