But to be a professional engineer. Study maths & physics at school, go to uni get a BEng or MEng. Then get a job in industry.
To be an expert, well you wouldnt be an expert in electrical engineering. You would be an expert in a very small area which is encompassed in electrical engineering. A couple routes. Go to industry and work in R&D in a niche area deeper then other engineers and become a sort of technical expert in one small area. or pursue a PhD in a niche area.