Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
Dear _[name of hiring manager]_,
In reviewing the job listing board this week, I noticed that you had a _[name of position]_ position open. I am very interested in the position, and would love to have the opportunity to speak with you about it.
I have been with the company _[length of time with company]_ and feel like I would be able to be a valuable addition to your department. I have enjoyed my time with the company, and have learned a great deal while working for _[name of current manager]_. However, I feel that I have learned all that I can regarding _[name of current position or department function]_ and am excited about the opportunity for learning new things.
I have notified my manager of my intent to apply for the position, and look forward to speaking with you more about this opportunity.