CELTA is a TESL/TESOL course, run by Cambridge. It's the most internationally accepted TESL qualification. You can do it at IH in Bangkok. It's not really necessary for teaching in Thailand, particularly as it is for teaching English to adults, not kids. But it's a great course, especially if you've never taught before. It's not cheap, takes a month to complete and is intensive. You have to produce lesson plans, teach observed classes (and accept the feedback from your peers) and do written assignments.
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