What to write about?
Something about yourself - what you study, what you do in your life, what your hobbies are, sports you do, maybe even what languages you speak... BEST courses are always a lot of fun, so the organisers should know that you are a fun person!
Why do you want to go abroad, and why do you want to go to a BEST event? It is important for the organisers to have internationally minded people.
Why do you want to go to THIS particular event you're applying for? Why is the topic attractive? How will the knowledge you gain there help you (in your studies, in your future career...)? BEST courses, for example, have a strong academical part (depending on the level of the course), so it's important for the organisers to know if you're actually interested in the topic! Naturally, if you're applying to a Leisure Event, then you can ignore this tip.
Explain why you want to go to that city or country so badly! It's strongly recommended that you do a small research of the place you're applying to before writing your motivation letters. Famous monuments, typical foods and drinks, traditions etc. Organisers like it when applicants show a real interest for the location they're applying to.
What do you have to offer to the event? In other words, why should the organisers pick YOU? (like: you're a star in karaoke, you can dance like Michael Jackson, or you know more than 100 jokes).
Sometimes, it is required to tell the organizers that you have a sufficient knowledge of the topic of the course. Read the course information on the site carefully in order to find out this sort of information.