Where to start?
Before applying to any event, you should read the information on the webpage very carefully. What will the event be like? What type of participants are the organisers looking for? etc. Reading the Survival Guide can be very useful in this sense!
Local BEST groups can receive something like four hundred applications to their course! Can you imagine reading all those different motivation letters? If you want your letter to stand out, then try to be creative and funny and capture the attention of your reader.
Needless to say, the letter must be written in English.