Writing an application letter

I decided to drop by the HR office this morning and was told by one of the HR officers that I should apply for a promotion considering my years of service in my company. I was very excited and rushed myself off to the office so I can begin on my application. As I begin to type, I was at a lost for words on how to start my letter. So, I browsed through the internet in a hope that I could come up with a very good cover letter. I'm still searching now..what to do, what to do...