Earn a bachelor's degree, preferably in education. It is best to have a four-year degree majoring in second language as you will be looking for teacher certification in the near future. Having four years under your belt is a good start.
Learn a second language. This is a very good idea as it will give you insights into how a second language is acquired, which will assist you in teaching a second language.
Apply to your state's education department with your qualifications; fulfill the individual state's requirements for obtaining a teaching certificate. If you have the necessary education methodology courses, it will likely be granted after some sort of basic skills examination.
Teach in the field. Gain experience by assisting a teacher who works in the ESL department of a school. If you don't secure work as an ESL teacher at first, work in the field of teaching to gain experience at class management. If your degree is not in Second Language Acquisition, teach in your major area.
Gain entry into a graduate school and pursue a Master's degree in Applied Linguistics, or TESOL in Australia and ESOL in the United States. With this graduate degree, you will be qualified to teach ESL in any country.