How to write an informal letter?
An informal letter is a letter you would write to a friend, a family member or an acquaintance.
The classic informal letter has five parts as follows:
1.Address BlockIncludes your return address and the date of writing the letter. This block is usually right-justified.
2.Opening SalutationThis is the opening greeting; typically "Dear John".  Left-justify this block.
3.Body BlockThis is your actual letter content (introduction + main parts + final paragraphs). It can be anywhere from a few paragraphs in length up to multiple pages. In the exam the length of the letter will be defined in the task (usually somewhere between 120 and 180 words). The body paragraphs should be left justified.
4.Closing SalutationThis is where you say goodbye. Typically, closing salutations for informal letters include such phrases as: "Yours truly,", "Your friend,", "All the best,", Take care,". These days, more informal closing salutations are also acceptable, such as: "See you soon,", "Don't be a stranger,", etc. Left-justify the closing salutation.
5.Signature BlockSince it is an informal letter to someone who knows you reasonably well, just sign your first name. Also left-justify this block.
For writing informal letters use informal English:
 a)Use short forms like isn’t, won’t, it’s, I’ve instead of is not, will not, it is, I have, etc. This is because we tend to use these forms when we are speaking, and using them in written English makes it sound more informal.
b)Sometimes miss out words in a sentence - In letter writing, it is often quite common to miss out the word ‘I’.  For example, at the end of a letter you might write ‘Hope to hear from you soon’ instead of ‘I hope to hear from you soon’. Other common expressions are ‘Wish you were here’ and ‘Having a great time here’.  Including these gives the impression of a letter or email being written quickly and informally.
c)Use informal intensifiers – In spoken English, we often use words like really and incredibly meaning really nice, really good. We often also use some of the more extreme intensifier + adjective combinations such as completely fantastic, absolutely awful, etc.
d)Use simple vocabulary - In English there is often a choice between  a simpler word or phrasal verb and a longer more complex word. In informal English it is better to choose the simpler form, so you would say get off the boat rather than disembark from the boat.
When you have finished, always go back and look at your work again. Don’t just read it three or four times – look for something specific each time.  For example, if you know you sometimes make mistakes with word order, check each sentence for word order. If you have problems with prepositions, go back and check each one carefully. Remember that grammatical accuracy is very, very important.
13 Liverpool Road,
Islington, London
N1 0RW
5th November 2010

Dear Paula,
Oh, how I missed being with you during the holidays this year! Everything in London is so different from Spain that I don't know if I'll ever get used to living here. I'm so glad that my father's job at the Spanish Embassy will only last until June. Then we will come home, and you and I can spend the summer on the beach. (I hope you'll spend your holiday in Spain, as usual.)
I've already made some very nice friends, but don't worry - no one could ever replace you as my best friend! After school, I sometimes go to my friend Fiona's house. She enjoys listening to the same music and watching the same clips that you and I like. In fact, Fiona and I have tickets for Bon Jovi's concert next month. I wish you were coming with us!
Life is very different here. Would you believe that we have to wear a uniform to go to school? It's awful! A blue skirt and blazer, a white blouse and a ridiculous checked tie, not to mention a pair of horrible, long, white socks. Besides, there is nothing to do in the evening. All the shops close by 4.30 p. m. and pubs are only open till 11 p. m.
That's all for now. I must start my homework for my English class tomorrow. The teacher wants us to write an informal letter to a friend and I don't even know where to begin! Please write soon and tell me all the news. I miss you.