In the first paragraph a cover letter should state the position that you are applying to and where you saw the position posted. Also, if there is a reference number you should include it as a company may have several posting for the same type of position.
In your second paragraph you should state your qualifications for the position and bullet point the skills you have that match those in the job description.
Then wrap it up with a paragraph stating you have researched their company and have a strong interest in being part of their team with a mention of an interview.
I know there is a tendency to tell the company that you are the best at what you do and they will regret not hiring you. There is nothing wrong with promoting yourself just know when to stop or else it sounds overbearing.