When writing an official business letter, it's important to keep your tone and language relatively formal. Note that ‘formal’ doesn't mean pompous or obscure: you should use words with which you are familiar yourself and which you can reasonably expect the letter's recipient to understand.
You should also avoid technical phrases or jargon, particularly abbreviations, unless you are certain that the person you are writing to will understand them.
Start your letter with the greeting Dear Mr (or Mrs, Ms, Miss, etc.) Surname.
If you don't know the name of the person you are writing to begin with Dear Sir or Dear Madam; if you don't know their name or sex, use Dear Sir or Madam.
If you know the person well or have a close working relationship with them, obviously it's perfectly appropriate to address them by their first name.
Setting the scene: begin with a heading alerting the reader to the subject of the letter, and in your first sentence draw their attention to the matter you're going to raise or discuss. For example, In reply to your question concerning … or Thank you for your letter of …, or I recently wrote to you about …
The main purpose: introduce your main point as early as possible. The aim is to communicate your request, proposal, decision, etc. in a clear, concise way. Once you have done this, you may want to give more details, perhaps adding further background or relevant facts
Winding down: before the end of a business letter, it's usual to provide a brief summary of your expectations or of what you foresee as the possible outcomes. For example, I realize that you will be disappointed by this decision …, or I hope we can meet next week to discuss the issue …, or I look forward to hearing from you.
The wording at the end of a business letter follows a standard format:
If you know the name of the person you're writing to, you should end the letter withYours sincerely.
If your letter begins with Dear Sir or Dear Madam, it should end with Yours faithfully.
Your own name should be typed out underneath your signature, together with your position (if you have one, and if it's relevant to your letter).
Sample official letter
Here's an example of a well-constructed official letter.