Firstly, you must have a CV or no employer will take you seriously, this must contain all your personal details, educational details and qualifications and career history, including work experience. Try also to point out your key skills.
Your covering letter then should be addressed personally to the contact at the company and should be short but not abrupt. If you have experience relative to the position advertised, it would be a good idea to mention it, not in details, but just saying that you are interested in this position as you have held similar positions before, or worked for a similar company, that kind of thing, at the end of the letter say that when you would be available for interview and that you are looking foward to hearing from them. Keep it as professional as possible, and not as though you are writing to a personal friend. Don't grovel or over do it when trying to impress, as long as all your information is on your cv, you should be fine. Good luck.