A Cover Letter should be typed, well-written and directed to a real person. Find out who the potential decision maker is and address the letter to that person. At least mark it for the attention of the HR Manager or Managing Partner.
Your opening paragraph should pique the interest of the potential employer. Say why you are writing. Name the position that you are interested in, how you learned of the opening or organisation and why you are interested in the company.
The second paragraph should create a desire to read further. Provide additional information concerning your education, experience, qualities and interests as they relate to the position. Expand on your CV rather than repeat the CV’s content. Tell the employer specifically what you have to offer the organisation and support your claim with proof/examples.
In your closing paragraph ask for action. Either indicate to the employer that you will be calling them within a specific period or desire the employer to contact you. Be sure to emphasise your appreciation for their time.