How do you become an ESL teacher?

I want to know how to become an ESL teacher that meets national standards (not just a state) in the US, and international for extra info if possible. I'ld like to teach K-12 and Adult students. Do they have ESL teachers in Universities? I wanna find out how much you CAN study being an ESL teacher. Not that I would study everything, but I would like to know my way around the subject a bit more (n_n)