Basic of a Resignation Letter. There's no need to sugarcoat or get creative in the beginning; just state the position you're resigning from and the effective date. While you probably shared with your boss your reasons for leaving, you don't need to describe them here__keeping it simple is perfectly fine.
Turning in your two weeks notice can be terrifying__because no matter how well you think you know your supervisor, you can never be 100% sure of his or her reaction. Will he be furious and insist on walking you out the door immediately? Will she beg you to stay, enticing you with an enormous raise? Or will he be completely civil and wish you the best of luck?