While in undergraduate school, major in something that pertains to criminal law

Keep up a good GPA

Study for the LSAT

Apply to a law school, find a law school that fits your needs and is geographically where you would like to be

Listen to your professors

Take prosecutorial clinics

Apply for internships at various District Attorney's offices

Study and take the bar exam (this exam is what gives you a license to practice law in your state)

Apply for jobs as Assistant District Attorney