Traditionally, you work out a design. First, figure out what you want the plane to do. You want a fighter bomber that can do a mach three intercept? Your design will differ from the plane you'd make if instead you wanted a freighter plane that could carry one hundred tons of cargo.
You've got to pick out your airfoils and do an engineering assessment on stresses, then design a body. Based on your requirements, design some wings that use the airfoil ou think is best for your purposes. Make sure you know what engines you'll use too so you can design them into it.
Once you've built it and installed avionics and everything, you can test fly it from some an accomadating airport (Mojave is good, they do a lot of experimental civilian aircraft).
