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。:)
2019-07-19 16:25
方兴航
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
戚姬顺
google it
you will get as many as you want