Dear Sir

How can write a letter for my uncle or my father. or the others

[Hi_there_Carl]

The format of an informal letter is pretty loose. Usually you begin with a date and then an expression of greeting or goodwill will like this:

December 16, 2007 (Date up near the top)

Dear Dad, (Salutation with a comma after)

(body of the letter)
I have been meaning to write to you for a long time. (put the rest of your message here)

Take care, Love, etc. (ending with comma after)
(your name at the end)