You need a degree and tefl/tesl. I read recently that they were cracking down on schools hiring teachers with no university degree. Don't come to Thailand without setting everything up first you will be screwed. Good luck!
Have you been here before? If not I'd do a test run first. The visa extensions and border runs are obnoxious... it's best to get help from reputable agency that willb help you navigate the annoying bureaucracy. You can get visa from thai consulate before you leave. Good luck!