Motivation Letter (Master's Programme in Management of Information)

Dear Sir/Madam,

I will be applying for master degree programme in the end of this year. It was required a motivation letter amog all documents required.
Since this is my first time writing this kind of letter, i would like to ask you for some help:

What should i write? What kind of information should i put on?
Is there a recommended number of paragraphs?
Where can i find corrects samples of this letter?

regards,
Felipe