First, let me say that is really exciting!
As for how to start, it depends on your teaching situation. Are you teaching children, young adults, or professionals? Is the setting in a language school, public school, or university? Is there a set curriculum with specific textbooks, or do you have the freedom to create materials?
After gathering information about what is expected, create a classroom management plan to give ourself an idea of how you will teach the course.
I recommend The First Days of School: How to Be an Effective Teacher by Harry K. Wong. It gives a lot of information about how to start teaching.