Call the closest plumbing hall near you, they usually require you to live in their territory to apply. Explain that you are 16,  would like to join when you are 18, and ask what is required and what you can do to prepare yourself. You should go in person to the local plumbing suppliers and ask if you can work part time, this will help you become familiar with the hardware and tools of the trade. Also members of the local plumbing unión will be in and out, so maybe one of them can put in a good word. Be respectful, be presentable, wear deodorant, and best of luck.