The resignation letter proper follows the ordinary business letter format:
June 20, 2006
Mr./Ms. (name) Proper salutation plus name of
Dear Mr./Ms. (name)
(your name) Same name as your signature.
cc: (name and title of H/R supervisor)
The "cc" is short for "carbon copy" (remember them?) and implies that the named person or persons will get a copy of this letter (a photocopy). Make sure you actually send it to them, in a seperate envelope.
The original letter goes to the person named as your immediate boss. Keep a copy for your own records.
If your employer doesn't have an H/R department, or if you're resigning from a volunteer post, then the "cc" and resulting copy aren't needed.
Each of this site's resignation letter templates includes a the letter pre-formatted as a Microsoft Word .DOC file.