What knowledge is needed to build a small aircraft?
One way is trial and error. That is how the early pioneers learned it, and building a few models is a good way to learn the basic stuff, too. That would be


Aerodynamics (what forces keep the aircraft in the air)
Flight mechanics (how to control it and make it fly straight by itself)
Structural design (how to make it light and stiff)
Systems (what equipment is needed to operate it)


My next recommendation might sound boring, but reading the federal regulations for light aircraft will cover all those aspects and more. In many cases you will wonder what that particular regulation tries to accomplish: This is your chance to learn! Post a question here and don't give up until you have fully understood why that particular regulation has been added. Yes, this will take a while, but building an aircraft all by yourself is no mean feat.

Last, join the local chapter of the Experimental Aircraft Association or whatever organisation supports experimental aircraft builders in your country.