In more rural regions of Africa and Asia, being a native English speaker may be the only training you need. But for most places, you'll also need at least a bachelor's degree, though your main course of study doesn't have to be limited to English -- it can be in anything.
In the U.S., to teach ESL in the public school system, you need a master's degree in linguistics or TESOL (teaching English to speakers of other languages). The same goes for teaching at most community colleges [source: The Linguist].
But if you're looking to volunteer, TESOL certification, also called a TESL (teaching English as a second language) or TEFL (teaching English as a foreign language) certification should work just fine. These programs are taught at many universities and community colleges, typically running four to six weeks to complete 120 hours of training in classes like linguistics, teaching theories and practical application [source: Learn 4 Good].
You can also take classes online, but be sure to take the courses from an accredited program, particularly if you want to work in the U.S., where you may also have to get a state license to teach ESL in public schools or colleges [source: The Linguist].
Most TESOL certification programs require you to have some college courses and maintain at least 2.7 GPA. The certificate can be used both domestically and internationally as proof of in-depth training.
And don't forget the soft skills that teaching English requires -- patience, flexibility and appreciation for other cultures.
For more on how to volunteer to teach English, see the links on the next page.