If you want to teach ESL in a K-12 public school, you need to be licensed. In all states, getting licensed requires you to have at least a bachelor's degree as well as a certain amount of supervised practice-teaching experience. Some states require that you have a master's degree as well.

As part of their licensure processes, many states require you to take an exam such as the Praxis series. The Praxis II exam tests your knowledge of the particular subject you want to teach - in this case, ESL. The Praxis III exam tests your knowledge of general education theories and teaching methodologies. Passing these tests helps show your competency to teach.

If you want to teach adults, there are many proprietary schools that offer ESL teacher training.