How Do I Write A Letter Declining A Request For A Donation?

You should write as under;

Mr. Name
Company
Address

Subject: Donation

Dear Sir/Madam,

Thank you for writing me a letter. Your organization is doing a noble cause to bring happiness in the lives of (you can mention here the function of the company, means for what they are asking for donation). I am sorry to inform you that my company is already donating to other organizations. I will try my best to send you some donation in the near future.

Thanks & Regards,

Your Name,
Address.