How To Start With The Covering Letter?

There are two types of covering/application letters:
Solicited
Always refers to the advertisement announcing the vacancy.Mention the source of the advert (newspaper or magazine, with date of publication). Give some details about your person and don't forget to state why you would be interested in working for that particular company. Try to "prove" this preference by showing that you have always been interested in that line of business. Don't overdo it, though, and don't be insincere. At the very least, show enthusiasm. You may start with:

Dear Sirs
Having viewed your advertisement posted in BusinessMagz, November 12 2007, I am interested in applying for the position as Accountant Manager.

and you may continue with compentencies related to the position you're applying.

Unsolicited
An unsolicited letter comes from people who are looking for a job and write to a number of companies, stating who they are, what they can do, and asking if there is or soon will be a vacancy for a particular type of job.
There are several ways of attracting attention in the opening paragraph. One of them is to mention the name of a well-respected person in the company, if you know such a person.
Another way (although a bit worn nowadays) is to start with a question or statement that might raise the reader's curiosity. For instance, many companies struggle with a shortage of people who could devote themselves completely to a specific task. The people with a permanent job are so busi running day-to-day affairs that little time is left for such tasks. A trainee (like yourself) could perform such tasks. Always try to stress the mutual benefit of hiring you.

GOOD LUCK!