Formal letters
Here you can see a document with some Rules for Writing Formal Letters in English ; also you can have a looka at this one with Useful Phrases.
Have a look at this layout:
Informal letters
You are allowed contractions, abreviations, exclamation marks, etc.
Use only your first name to sign. After all, you are addressing a friend.
Optional: the sender’s address and the date are optional
Don’t write the recipient address.