IELTS Task1 Letter "You have just arrived in the city "

Question
You have just arrived in the city where you are going to spend two years for your master's degree. You have a lot preparation to do before the new term stars, but you do not know how to go round the city. A friend you have newly acquainted offered you guidance and helped you through difficulties.Write a letter to express your thanks.

My Answer;

Dear Sir,

I am Andy,the student you helped last week.I am writing to express my appreciation for what you have done for me.

Last week,I had just arrived at the city.Obviously,I was not familiar with this city at all.I was preparing for my new term.I needed to get my package at the post office and buy some books for my class.However,I did not know how to get around this city.Fortunately,I came across you.Thanks to your great help.I went through all of the difficulties.

According to your directions ,I found the post office and got my package.In addition,I bought the books and a map in the bookstore.I followed your advice and got familiar with the bus stations around the school.Otherwise,it would be very difficult for me to get ready for the class.I would like to invite you to dinner next week.I sincerely hope you could come.I have other interesting news to share with you.

I am looking forward to hearing from you.Thanks for your kind help,again.

Yours
Andy
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Hi guys ,I have a couple of questions about this letter; Could you help me please, Any suggestion and correction would be appreciated.
Thank you very much indeed.
1. The question is "A friend you have newly acquainted offered you guidance and helped you through difficulties." ,Do you think it is all right to write "Dear Sir," as the beginning?

2. Do you think it is all right i write like this"Otherwise,it would be very difficult for me to get ready for the class." Because it looks like not very good. Precisely,I think the word " Otherwise " is a little wired.

... You need to apply the appropriate tone for this letter... The prompt says that a friend (newly acquainted) offered you help. So you need to address him more informally. Say;

Dear Tom
If you are writing to a professor or some official or even an unknown person, then you must address them in a formal way.


I am Andy and hope you remember me. I am the one that you helped last week to get around the city. I am writing to you thank for your thoughtfulness and generosity and had you not been there I would have not managed half what I intended.