TEFL is the abbreviation for Teaching English as a Foreign Language. This term is mainly used in UK. Sometimes it also refers to teaching English to non-native speakers in a non-English speaking country.
ESL is the abbreviation for Teaching English as a Second Language. Mostly used in USA. Additionally it is used to describe teaching English to non-native speakers in an English-speaking country.
No, you do not. If you plan to get a TEFL certifacate, teaching English with Thai-Experience can be a good preparation as you will gain practical teaching experience. And at the same time you can find out if this is an option for you, how things work out for you and how you react in a total different environment before making a longer commitment. What you need for your volunteer work is an open mind and a good command of English.
There are many places where you can get a TEFL certificate. You do not even have to attend in person. Some institutes offer online courses which you could take while volunteering and gaining teaching experience.
Thai-Experience.org does not offer TEFL certificates or courses.