I would highly recommend this course

If you are interested in teaching English as a second language, or think you are, then the Online TEFL Starter course is for you. The course addresses all of the primary facets of teaching, learning, and the English language, and distills them into easily digestible pieces.

These pieces, coupled with the expert guidance of the tutor, give you a clear idea of what to expect from the full Cert TESOL. I would highly recommend this course.  

Ed Miller