The greeting depends on who the letter is being written to. The most common greetings are:
The introduction is where you introduce yourself (recipient is unknown), say why you are writing, and explain your situation.
The body is the main section of the letter, where you give further details. Your letter should be divided into paragraphs and, as in an essay, each paragraph should have one main point.
The closing or sign-off is where you finish your letter and request a response, if appropriate. Like the greeting, the sign-off you choose depends on who you are writing to.