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 [name position].
I am interested in starting a new career. With my previous experience in the [type] industry and this position having many similarities, I believe that my skills would be a good fit. I learn very quickly and am willing to obtain any necessary education to do an excellent job for you. I take direction very well, I work well with others and I am also able to work independently. I would enjoy being a part of a leading team. [note: it would help to address some points in the job ad here. i.e. if the company wants a team player, say you're a team player.]
Thank you in advance for taking the time to review my enclosed resume and I look forward to meeting with you in person. You may reach me at [phone #] during the day or [phone #] evenings.