See more information about how to get started at the Central Wikia tutorial.
So you're going to be teaching at elementary school (小学校 shōgakko?)? Congratulations on landing one of the most rewarding and challenging experiences on the JET Programme! Everybody knows that Japanese kids have a monopoly on cuteness and you'll get plenty of opportunity to experience that first hand. That's not to say that it will be easy! More than likely you'll be teaching 4-5 classes per day and will feel that you definitely earned your money at the end of the day.
As with everything on JET, Every Situation Is Different but you'll have a bigger teaching role in the classroom compared to JHS or SHS because elementary teachers don't speak English and have no experience of teaching it as a subject. This is great if you've got teaching experience yourself and will allow you to utilise your talents to the full, but can be a bit daunting if you don't have a clue what or how to teach.