Hi my Q is I have just started a mall advertising consultancy in dubai and I want to writ a wining introduction letter to my previous clients in other agencies I worked for can you advise me on how to do this or give me examples for such letters

You will find examples on the following links.

Introduction Letter Example - Google Search (http://www.google.com/search?hl=en&q=Introduction+Letter+Example)

Introduction Letter Sample - Google Search (http://www.google.com/search?hl=en&q=Introduction+Letter+Sample)