Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
Please let this serve as my official notice of resignation from [name of firm]. [date] will be my last day of employment.
I would like to thank all of you for the opportunity that was given to me, [#] years/months ago. During my employment here at [name of firm] I have made numerous [partners/friends/colleagues etc.] and I thank them personally.
This wasn`t an easy decision for me to make. It has been a pleasure working here. Thank you and best wishes for the future.