I have been an ESL teacher for the better part of the past 10 years. When starting out, you should determine which level, and which age, of student you are most comfortable with. Lesson planning, classroom management, even your personal demeanor when dealing with students can all be taught, by a reputable organization or institution, but if you're comfortable with advanced preschool students, and you need to teach beginning adults, well, that can be learned, it just takes time. When you are starting out you should have as few challenges as possible. A really good course will cover this. I got my CELTA training from Embassy C.E.S. in New York. They have centers, sorry, centers, worldwide. I highly suggest you seek training from them. Best of luck, and I hope that you find teaching as rewarding for you as I have, should you choose to follow the Way of The Teacher..