How do I write a very persuasive cover letter for an entry job in advertising?

There are plenty of sites devoted to the subject of how to write cover letters. If you click on the following link, you will find some a lot of them.

Writing Cover Letters Sample - Google Search

Now, in terms of us being the most helpful to you here, if you could post a draft of what you are thinking of writing, then we could look at it and give tips to you as to how to make it better.

Just for an example, I like to start mine something like the following:

Dear...

Please accept this correspondence as an indication of my interest in the position that you have open for (a, an)... I believe my qualifications for this position to be strong.

[Then I go on for a couple of paragraphs describing how and why I might be the best person suited for the position as well as some other things that I might have to offer to them or experience that might not already have been mentioned in my resume that might be specific to the type of position that they are wanting to be filled.]

[Closing] Please find attached a copy of my resume. I look forward to an opportunity to discuss how I might serve the needs of...

Sincerely,

[My name.]