The most commonly accepted qualifications are the Certificate of English Language Teaching to Adults (CELTA) awarded by Cambridge ESOL and the Certificate in Teaching English to Speakers of Other Languages (Cert. TESOL) awarded by Trinity College London. These certificates are generally seen as a minimum qualification to teach English as a Foreign Language.
Distance learning courses can also be a good introduction, but feedback on your teaching practice is essential and most distance courses will not include this, and therefore will not be acceptable to many teaching institutes.Cambridge ESOL have recently launched (2011) an Online CELTA which blends distance learning with teaching practice in a classroom. Visit the Cambridge ESOL website for more information.
The CELTA and the Cert. TESOL can be taken full-time or part-time at centres worldwide. The centres undergo a strict external inspection to guarantee standards both of the course and of assessment, and the courses include a substantial proportion of observed teaching practice on real students.