Well, there are many ways. You might inherit powers from a long-lost relative or get chosen by a powerful entity to receive supernatural abilities. Another possibility is that you train really hard and develop unique skills that make you a superhero.
Often, it involves a combination of events. Maybe you discover a hidden artifact that grants you powers, or you have a near-death experience that triggers a transformation. Sometimes, it's just a matter of being in the right place at the right time and making the heroic choice that leads to your superhero journey.
It could be through a sudden accident that gives you extraordinary powers. Maybe a radioactive explosion or a strange chemical concoction.