How can I design and build an aeroplane from scratch?
Take the example of Ed Swearingen: He never went to engineering school, but spent years working with airplanes, gradually moving from handling to repair to improving them. In the end, he was a successful self-employed consultant who helped with a number of airplane designs and created some of his own designs, too.

So here is what I propose to do:


Hang out at a local airport (ideally one with lots of hardware passing through the local repair shops)
Don't shy from getting your hands dirty and from asking questions until you are satisfied with the answer.
Try to find experienced engineers and work closely with them.


Do that for 10 years and you will be hard to beat as an aircraft designer. If you happen to attend University in Germany, be sure to become a member of one of their Akafliegs - that is the best start into an aviation engineering education.

If you have already some idea how your design should look like, build it as a model first. Models can give you valuable lessons in aerodynamics and flight mechanics.