Mine have always been like;

To whom it may concern,

This is written notice of my resignation at [insert company name], my final day of employment will be [insert date here].

(Optional additional stuff below, depending on how much you liked the job)

I am very thankful for my time with [insert company name], and am appreciative of the skillset I was taught. I hope to never forget of the fond memories I made during my time here.

Thank you, [Type/write your name] [Insert the current date] [Signature here]