Daniel Cassany is probably one of the best authors in the world about teaching languages. His work is focused on teaching both a mother tongue and a second language, and is especially aimed at teaching writing skills.
He is also one of the best investigators in the European Language Portfolio.
Look at his web and enjoy learning how to teach languages. You will be able to apply any of his articles, books and research to your lessons.
You can start by reading a bit of his essays. For example, this on-line article about writing compositions.