Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear Sir or Madam:
RE: [name of position]
I am writing in response to the ad that you had in the [source of ad] dated [date] for the position of [position name].
I believe that I would be an excellent candidate for this position based on my previous experience and my willingness to take on new challenges. Having worked [#] years in the [type] industry has given me a strong background in the [type] field. I am confident that I would be able to make a significant contribution to your organization.
I have an great work ethic and believe strongly in professionalism. One of the reasons that this position is so appealing to me is because of the challenges it presents. I am always up to a good challenge and am willing to take any educational courses to help me do my job better. I'm also a great team player and enjoy to work with others. [note: it helps to tailor this section to the job ad. i.e. if they're looking for a team player, say you're a team player.]
Thank you in advance for taking the time to review my enclosed resume. I look forward to meeting with you in person to discuss this exciting opportunity that your company has available. Please feel free to contact me at [phone #] during the day or [phone #] evenings.