Your Contact Information
Date

Employer Contact Information
Name
Title
Organization
Address
City, State, Zip Code

Salutation
Dear Mr./Ms. Last Name,

First Paragraph
Your letter should say that you are resigning and state when your resignation is effective.

Middle Paragraph
The next (optional) section of your resignation letter should thank your employer for the opportunities you have had during your employment with the company.

Final Paragraph
Conclude your resignation letter (also optional) by offering to assist with the transition.

Close
Respectfully yours,

Signature
Handwritten Signature
Typed Signature

Source: http://jobsearch.about.com