They can have the intimacy of a journal entry, but with a particular audience in mind--vocabulary, quotes, anecdotes, all chosen to tickle a particular ear. They can send an "I love you," and all the reasons why, to be opened again and again--words which don't need an occasion to be heard, but which can be secreted away in backpacks to be read on the train, or pulled out in some other lonely moment. They can reach across the universe to extract the truth between two people, in that cathedral where there is only our voices--yours and mine, writing--a cathedral between side-by-side note-passers in class, or between friends across continents. It is a private, precious language, as intimate as a whisper. We pass a metaphor back-and-forth. We work on our masterpiece models of the universe and of the human heart. And no one is embarrassed by extravagant endearments. If we are, contrary to Steinbeck's suggestion, each his or her own island, each with an inner life never entirely communicable, then writing letters is, I believe, the closest we ever come to filling the space between us.
Listen to your deepest voice. That is the voice which must speak, unrestrained, on paper. Describe the lady at the post-office. Describe the ways in which you feel masculine and the ways in which you feel feminine. What are you afraid of and why? What would you do if you could do anything and why? Think of a person that you love, think of the details of that person, and then express your love for that person in terms of their details.
To get you started, I recommend Nick Bantock's Griffin & Sabine books. It is the fictional correspondence of two lovers across time and space. Not only is their correspondence interesting and intimate, but it features original postcard paintings by the author, and envelope flaps with pull-out letters. (Please note that these books are not meant for children.)
But these accoutrements, although fun, are not essential. The important thing is that you write. Most likely, you will be famous someday and someone will want to publish your correspondence. But even if there is no enterprising and favorable editor, there is probably a backpack somewhere which would be much improved by the addition of your letter. And there are many cathedrals which await your unique embellishment.