That depends on which organization you go with and/or the institution at which you work. Requirements vary by region of the world and by sending organization. Common ones include any combination of the following:
- A bachelor’s degree in any field
- A degree or certificate in TESOL, TEFL, or TESL
- A master’s degree in any field
- Previous teaching experience
- Physical abilities, such as climbing stairs (no joke!) or being under a certain age
- Some regions have other, unusual expectations (particularly in less stable regions of the Middle East and Central Asia, where requirements may include more demanding physical abilities, including the ability to run upstairs wearing body armor [not kidding!]).
Some sending agencies will help you meet their own requirements by offering certification and experience prior to your departure.