Dear (insert last name)
Paragraph 1: Introduce the reason you are contacting them, by explaining about your organisation or charitable company, and explain that you would like them to donate to your cause.
Paragraph 2: Go into more detail about their potential donation, explaining what benefits it would bring to both your company, and them, if there will be any publicity involved.
Paragraph 3: Explain the things you are doing to raise funds and awareness for your campaigns so that they see that you are an active organisation.
Paragraph 4: Explain what the overall goal of the campaign is, so that they will understand what their small contribution will go towards as a whole. Finalise this last paragraph by again asking for a donation, and by thanking them for taking the time to read your letter.