Formal letter structure

First paragraph
The first paragraph should be short and state the purpose of the letter- to make an enquiry, complain, request something, etc.

The paragraph or paragraphs in the middle of the letter should contain the relevant information behind the writing of the letter. Most letters in English are not very long, so keep the information to the essentials and concentrate on organising it in a clear and logical manner rather than expanding too much.

Last Paragraph
The last paragraph of a formal letter should state what action you expect the recipient to take- to refund, send you information, etc.