You need to be TESOL qualified in order to teach, although some Asian countries take in teachers without professional qualification if you are a native speaker but most jobs require you to have a TESOL certification or an equivalent.
A number of institutions all over the world run TESOL courses and generally all you need to enroll yourself in a course is the ability to produce some University level work. I cannot help you to choose which course would be best for you as I'm not very informative but I did my course from TEFL International in China (Zuhai)and I think my money was well spent. I'm now teaching in China for the last two years and having a ball. Cheers!!