After I desired a shift in my career path, I applied to the program because it seemed like a great, immersive, but short-term opportunity. It turned out to be an excellent decision, and I have no doubt it will be for you too. CIEE is there with you every step of the way and streamlines the process - so whether you are a first-time traveler or a seasoned traveler, you'll be confident getting on that plane.
You can choose your preference for what age group you want to teach, so the sooner you apply, the better! You will likely teach a mix of different grades, which makes for a lot of fun. As far as how much "lesson planning" - that's up to you and the teachers. For some classes, I would propose things via email to teachers (like cultural presentations and games about Thanksgiving, etc), and they were thrilled. Other teachers preferred me to assist, rather than to lead. Both sides of this were really valuable, however!