FORMAL LETTER FORMAT
Salutation ( don't use dear sir it will be informal)
Body of the letter containing introduction and conclusion
End your letter by writing yours sincerely, your faithfully but please don't be informal and write yours lovingly.
INFORMAL LETTER FORMAT
You can write this informally as your best friend or your lovingly etc.
remember one thing that don't use abbreviation in the formal letter. Just make the letter formal by the formal words such as don't use "you can' always use "you may". Also date should be in the proper format as "24 August, 2015".
One more thing that every letter of the subject in the formal letter must be capital.