Think about who and where you want to teach. TESL and Teaching English as a Foreign Language (TEFL) are often terms used interchangeably, but they actually have different meanings. Where TESL involves teaching English to non-native speakers in an English-speaking country, TEFL involves teaching English to non-native speakers in a non-English speaking country. You therefore need to think about whether you want to teach English abroad in a non-English speaking country, abroad in an English-speaking country, or at home. If it's the former, you want TEFL; if it's the latter two, you want TESL.
Find the TEFL requirements. If you have ascertained that you want TEFL, then you need to find a school or location where you want to do this, and find out what their requirements are. You can find jobs and placements on websites such as i-to-i (onlinetefl.com/tefl-jobs-abroad) or Go Overseas (goteachabroad.com). You may find that some placements do not want a qualification at all and just want a native English speaker. Others may stipulate that you are a trained English teacher, while others may ask you to complete a TEFL course.
Find a TEFL course. If the placement or job you want to go for requires a TEFL qualification, then you need to find a course provider. There are various courses available, from classroom-taught classes to online and distance-learning classes. Some can be completed as a weekend intensive course and others take four weeks or 130 hours. Double-check that the TEFL course you found meets the requirements of the placement or job you want. You can find TEFL courses listed on websites such as English Club (englishclub.com) or on i-to-i (onlinetefl.com/tefl-course).
Find the TESL requirements. Alternatively, if you have ascertained you want TESL, then you need to find the school or education center where you want to do this and find out what their requirements are. You can find jobs and placements on websites such as ESL Teachers Board (eslteachersboard.com). Most TESL jobs will stipulate that you need to be a trained teacher already or have completed a TESL course.
Find a TESL course. If the placement or job you want requires a TESL qualification, then you need to find a course provider. As with the TEFL course, there are a number of courses available either online or in the classroom. You can find TESL courses listed on websites such as ESL Base (eslbase.com) and i-to-i (i-to-i.com/esl/online-esl).
Complete the course and start teaching. Once you have completed the course and passed the ongoing assessments, you are ready to teach ESL or EFL.