That is a question for your local American Legion or the national Boys' State organization. Schools and ALs vary in how they come up with the nominees. My sons' school used a panel of teachers and other schools let guidance counselors make the selection. Make some calls and do a little digging.
Any one can nominate you, however it best if it can someone who is aware of your leadership abilities. You will have a good time. And if you are picked or elected I think you qualify for Boys Nation.