Hello, I hope you are asking about the statement of purpose and it should be written in a narrative format but not in letter structure. You should start by first mentioning your academic and co curricular qualifications and then mention your related courses, projects, workshops pertaining to that field  justifying how much willingness you show towards joining that course. Then you should add what kind of experience you are expecting to gain from their institute and what you will be giving back in return-it should pertain to your career goals that you want accomplish in future.

Hope this helps.