Letter of Motivation

Hey there,

I'd like to do a gap year and therefor I have to write a letter of motivation, but I don't know how to write it. I put one my thinking cap and asked google a lot, but it just spit out information how to apply for universitys. I don't even know how to begin properly because all the letters I've read start with something about an university. May someone just tell me how to begin properly so that I can start. Just some brainstorming. I really appreciate that.

greets

PS: I am about to finish my A-Level this year. So I got no special knowledge.