Dear (insert last name)
Paragraph 1: Introduce yourself, and explain that you work for a charity and tell the reader that you are looking for sponsorship and indicate why they have been contacted as potential donors.
Paragraph 2: Explain a little bit more about your organisation. Tell them who or what you help or support and what their money could be doing for your charity.
Paragraph 3: Give some more information about your organisation in terms of any events that you are holding to raise awareness, or anything else that crops up in your social calendar.
Paragraph 4: Explain how much funding is needed, and exactly what this amount of funding will be able to do. You can then ask for a specific donation from the reader. Finish by thanking the reader for their time.