Motivation Letter

Hey all,
my name is Matthias (german for of Mathew) and I am in 12th grade. I am applying to the Hague University for a spot in their International and European Law bachelor's program. As part of the application, they've asked me to write a motivation letter. My problem is that I have never written a motivation letter. Is it just in the form of "I want to go to your school, because..." or is there some more extensive format to the whole thing???

Please help me!

Thanks in advance,
Matthias