You can read about the Youth Ambassador (YA) program here. Basically you come into the program through one of Inspire/ReachOut's 'special projects' - things like the youth moderator program, youth editorial board, ReachOut speaker's bureau, etc (there's a whole range of projects that young people are involved in). Once you've come through the initial program, you've got the opportunity to become a YA. It doesn't look like Inspire/ReachOut are currently advertising any special projects, but if you're keen keep an eye on this page, where special projects will be advertised when they come up.
Gail has answered that perfectly, I just wanted to add that it's a great experience and always fun being a YA, so I too would definitely encourage you to get involved when the next special project is run (or the next one that sparks your interest)!
Hopefully something comes up soon, keep your eyes peeled