I need to write a essay on why I want this job.
There are various ways to approach an essay like this one. Here is one method.
In an essay of this sort, you will want to describe three things: the job, the nature of your interest, and yourself.
Without having worked the job before, your ideas of what exactly the job will entail may be imperfect. However, your vision of the job is important to the employer. The first task in your essay will be to express you ideas about 1) what the job entails and 2) how the job fits your plans, ambitions, interests.
Your second task will be to articulate the connection between how you see the job and how you see your future. Alternatively, you can articulate how the job fits a specific interest of yours or fits a specific skill set you possess. This is the section where you will express a view of where you are now and where you would like to go (professionally). Answer the question, "How does this job fit your personal plan?"
Finally, offer a brief articulation of who you are. You might mention how you developed the interests and ambitions from the second section of your essay. You might mention things you have done that relate to the job.