Motivation letter for high school graduate

Hello everyone,
I'm a high school graduate and I'm applying to a business school in Sweden. I find it very hard to write a letter of application because I have no work experience nor any experience related to the field I'm applying to I often see that a motivation letter is more of an requirement t for people who are applying for a master degree?
I would like to have some guidelines on what I should write in my letter of motivation. I understand that it's not a cv but more of a personal letter? How do I keep it not too formal but not too inappropriate?
Thanks