If you plan on competing, you must register through USA Boxing. Make sure to schedule a physical appointment with your doctor, if you have not had one within the past year, because it will be needed to complete registration; the paperwork for the Athlete Physical to be signed by the doctor is available here (if you've had a recent physical already, the signature page can generally be dropped off or uploaded to an online medical chart for the doctor to sign).
Also, if you would like to become a volunteer, you must register on USA Boxing and pass a background check.