Although a few schools in some parts of the world may require only that you be a native speaker, most institutions prefer to hire teachers with a 4-year college degree and some TEFL training. In an increasingly competitive environment, it's an absolute necessity at some schools. In any case, having a CELTA or TEFL certificate on your resume will make the difference between you and another native English speaker, and can increase your chances of working at a better school and/or receiving a higher salary.