The path to becoming a criminal justice lawyer is a process. Law school is a first step, and you can focus classes on criminal justice topics; including trial practice, evidence, and criminal law. Most law programs are 3 year programs after college is complete.
During and after law school, you can enroll in legal clinics for courtroom practical education, or find internships with the public defender's office or District Attorney in your area. These are hugely helpful if finding out how legal work actually gets done. These are some of the best steps to take toward becoming a criminal justice lawyer. You can also find part time jobs with law firms that practice criminal defense.
I hope this helps, good luck on your career path.
You should now focus on the steps to getting into law school such as taking the LSAT and applying to the law schools of your choice.
Good Luck and Best Wishes.
Apply to law school. Take as many criminal classes the school offers.
After your first year of law school try to work or volunteer at either the law school law clinic or the legal aid to get some practical experience. After your second year try to get a job or volunteer at the district attorney's office.
This is if you are complete law school in 3 years.