Hello Valentin,

In modern English it is acceptable to write "Dear Sirs" and mean everyone who reads the letter. I've made some corrections below.

An Application Letter

Dear Sir or Madam,

I would like to apply for a sales assistant post with French Dress Corporation, which was recently advertised recently.
I have always have been a fashion enthusiast and if you look at my resume, which I have enclosed in the letter, you will see that I have a great deal of experience in clothes sales the fashion retail sector.
I have need to mention though, that I am not available immediately because my current contract of employment in my current company requires one months notice.
I feel that I am the ideal candidate for this position and I strongly believe that French Dress Corporation and I will mutually benefit from our cooperation.

Looking forward to hearing your reply in due course.
Yours Faithfully.
B. L.