It varies but for me, I followed what I was always most interested in school. Science was always my favorite subject. In college I had to complete general education courses like History and English but my interest in science made taking the "boring" classes worth it. If you're not sure where you want to start, I would recommend taking general education courses at a community college to see what you enjoy the most. Or if possible, volunteer in fields that interest you the most.
After you find what interests you the most, you can begin to research careers you're able to apply for using your chosen field/major. For example, I loved learning about bacteria and looking in a microscope. I tried finding careers that would allow me to do both. I wish you luck and happiness in your career search Nick!