google it
you will get as many as you want

You can write anything that clearly indicates you decided to leave
最简单就写"I resign"也可以。当然，一般都会加上一些客套话的，诸如我还是很喜欢现在的公司，但是（以下若干字）等等。:)

A sample
Dear Mr. xx,
I write to confirm that I am resigning from the position as xxx.
This was not an easy decision and took a lot of consideration. However, I would like to pursue a more challenging job for my career development. I have learnt a lot from you and other colleagues during the past several months. And I enjoy the time working with you.
My last working day will be xx. Please be assured that I will do all I can to assist in the smooth transfer of my responsibilities before leaving.
I wish you every good fortune and I would like to thank you for having me as part of your team.
I look forward to hearing from you.
Yours sincerely,
xxxx