Letters to friends and family should be written in a conversational style. They are a bit like a friendly chat. They should be spontaneous compositions and as such they should be unpremeditated. Compared with essays, social letters have a free style. When we engage in a friendly conversation, we touch on many topics. In the same way, you can write about just about any topic in a friendly letter. You can also present them in whatever order you like.
In friendly letters, you can use colloquial expressions. They would look out of place in an essay or a formal letter.
This, however, does not mean that you can be careless. It is insulting to ask a friend or a family member to read an ill-composed letter full of nonsense. So, even while writing an informal letter, we have to persevere to present our thoughts in a logical manner. You also need to ensure that you follow the rules of spelling, grammar and punctuation. Spelling and grammar mistakes will at once give the impression that the writer is an uneducated person.
In friendly letters, you can use a salutation beginning with Dear or My Dear. Note that titles are not used in the salutation.
My Dear Annie
But if you are writing a friendly letter to a person who is much older than you, using a prefix like Mr. or Mrs. is considered as respectful.
The forms of subscriptions are varied. Common expressions include: Yours affectionately, Kind regards, Best regards etc.