Outside of schooling and passing the bar, the best way to become a criminal defense lawyer is to internship or externship with a Public Defender’s office or Prosecutor while you’re in law school. This is for two reasons, first, to make sure you know what you’re getting into and if you can handle it. But more importantly, you’ll get tremendous amounts of in-court exposure which will pay off in spades across your career. Prosecutors and defense attorneys are counter roles to one another, so you get to see how they operate and the volume of cases you’ll handle. Public defenders handle large caseloads for the most at need who don’t have defense representation, so that’s equally valuable.
I was a criminal law intern at two private practices for 5 years while in undergraduate school and law school and then spent 2 years as a Juvenile Public Defender with the Public Defender’s Office. This was all strategic to give me the exposure and experience I needed (and wanted) to open my own law firm and go into business for myself. If you don’t do that, you can still become a lawyer, but without that courtroom experience, I suspect you’d have a very overwhelming experience once you got thrown into the fire, which is why almost all of us do the recommended route.