I would suggest thinking about the why, how , what .
What parts of the organisation ( research it ) attract you
How do you see yourself fitting in to the organisation and where.
Why do want to be a part of the organisation
What do you have to offer that fits in with the organisation
Research the Mission statement and Organisational Strategy of the organisation in question.
Research opportunities for new employees in the organisation
Once you have the logical break down you can start with a brief statement on why, how and what and then further elaborate on what each logical part has and conclude with how it all fits in with you and the organisation.
Research Research Research !!
Research how to write an essay at University level.
Google it or go to the university library or your University student services in your university may hold courses also on how to write essays or have books available.