If you pass the St. John Ambulance Emergency course you can wear the badge. If you pass the St. John Ambulance Standard course you can wear the badge. (assuming that you are a cadet)
Show your CO your certificate. Badges are usually distributed by St. John Ambulance, but I believe that you can also get them through you cadet supply.
You may have to pay for the course, depending on who is giving it and where. My cadet unit does the course for free as we have several available St. John Ambulance Instructors.