HI, I am from the Czech Republic and I have just become an ESL teacher here in the USA. You have to fulfill certain criteria:
1. Have at least 30 credits from English
2. Have a degree (BA or MA)
then if you have these it depends on the state you are in, here in NJ you have to
3. Pass OPI (Oral Proficiency Test) and WPT (Written Proficiency Test)
4. Send all documents and application+fee to the Department of Education
If you are not sure what to do and if you don't know if your credits are enough, you can visit county educational department and ask them to help you out. Department of Education can also give you evaluation of your credits and diplomas and determine the area you can teach.

It is possible to go to some foreign countries to teach English without any qualifications besides being a native english speaker and a high school diploma. If you are serious about a career as an ESL teacher, then I do not suggest this way. I think having some maturity and training are necessary. As a high school graduate, without working experience, I dont think you will have the skills needed to gain the respect of your students.

There are more opportunities if you have a BA or BS degree in any major. I think majors like communication or English are good ones to pursue if you are intersted in becoming a ESL teacher. It also helps if you study a foreign language (doesnt matter which one, but in order to teach Englishassuming you are a native speaker I think it helps to be able to understand some of the difficulties and frustrations in language learning. BUT you do not have to speak the same language as your students) Some teaching methodology courses would also help you especially if you dont plan to get certified as a teacher and want to teach overseas. In fact, you can probably get a better job overseas if you are teacher certified and you want to teach K-12 in an international school. Anyway, after you have a BA degree you can apply for programs like the JET program to go teach in Japan. (But this is actually considered EFL).

If you are serious about being an ESL teacher, I would suggest trying to do some tutoring for helping non-native speakers improve their English.

You can look into some MA TESOL programs (Masters in Teaching English to Speakers of Other Languages). Other people choose to just get a 3 month TEFL cert. I think these are not useful if you want to teach English in an English speaking country.

As an EFL/ESL teacher, I can tell you that I am very happy with my career choice. Some of the benefits I get as a teacher are seeing my students improve their English and learning about different cultures. I really enjoy teaching college students and because English is just a tool for them to access more knowledge or further their career, I often can learn a lot from students because they have such diverse backgrounds.

A downside to this career is that the pay is not always very high and it can be hard to find a job in certain areas (especially a full time job).

I highly suggest reading DAVE'S ESL CAFE for more information. The web address is www.eslcafe.com . You can also check out the professional organization for ESL teachers. It is called TESOL, www.tesol.org .

In most states, you need to become a certified teacher, and then earn an extra certification in ESL. Check into undergraduate and/or graduate programs in education. When you ask questions about getting certified for ESL, you will get answers. Also look on state websites for the licensing of teachers. They will list the requirements needed in your state, or the state in which you would like to teach.

Besides getting certified, you can also teach English online. There is no qualification, but it does help if you're a native speaker. if you're interested, the following website is a step-by-step guide for teaching English online, and includes lesson plans, activities etc. to get you started.