Undergraduate / Application letter to Mission College [3]

Hi there,
I plan to apply to Mission College, and I have a question about the application process.
They tell me to write "A handwritten letter that explains why you want to attend Mission College."
But I dont understand what form I need to write; should I write it as an essay or just a journal? Please help me figure out that and if possible, can you give more some ideas what I should and shouldn't write on that letter. Thank you very much