What is the process for retraining in a different specialty, or transitioning to a non-clinical career?

I am a year 10 high school student, hoping to pursue a career in medicine. I am interested in becoming a paediatrician, but am curious how I would be able to specialise or transition to different areas within working clinically and non-clinical fields. For my year 11 subjects I. am doing, chemistry, biology, psychology, math methods and energy systems. I will continue on tp do these subjects in year 12 as well.