Visit a local tattoo and piercing shop in your area. Talk to a professional piercer there and explain that you want to learn how to pierce tongues. If they have the space, the studio may allow you to become an apprentice.
Watch a professional piercer in action if you are granted an apprenticeship. Watching the piercing taking place, and finding out what tools and actions are necessary is one of the best ways to learn.
Obtain information on piercing schools. You will have to take courses to learn the craft of body piercing, and it is the only way to earn a license toward your new profession. Talk with professional piercers in your area to get some ideas.