How to write a course application?

I’m currently put in Standard level economics class, and to get into Honour Economics, I have to write an application stating the reasons why I want to get into Honour Economics, and since I’ve never been asked to write an application before, I was wondering if anyone can help me out.

* I attend an international school, but since I’m a native speaker, I can choose to be in Honour Econ, I just have to write the said application. :/ i dont’ know how to start.