Most schools will require that you have a minimum of a bachelor's degree regardless of the institution's location. While some schools do not indicate a preference for the subject of your degree, others require or strongly prefer your degree to be in a related field such as English, education or applied linguistics. A graduate degree is typically only required for those interested in teaching ESL at the college level, but can also help you negotiate a higher salary.
In the U.S. it is generally expected that you will have one or more semesters of teaching experience through either your bachelor's degree or as part of another ESL course you have completed. If you are interested in receiving emergency certification (which usually requires taking a test in lieu of student teaching), it may help to tutor or teach private ESL lessons to add real teaching experience to your resume.
There are a few different internationally recognized ESL certification courses, including the TEFL (Teaching English as a Foreign Language), TESOL (Teachers of English to Speakers of Other Languages), and CELTA (Certificate in Teaching English to Adults). While very similar, each course is divided into lessons that focus on different aspects of being an ESL teacher. All of these courses are available to take at several universities, and many have accelerated or intense courses available. Courses also are available online.
Some ESL courses end with a standardized test that you must pass before receiving certification. For ESL teachers in the U.S., each state has its own standardized test that focuses on teaching ESL in accordance with that state's educational guidelines. To see the requirements and material of your state's standardized test, visit your state government's official education website.