Simply replace the unfilled areas such as "" and "[date]" with your information.
I wanted to let you know that I will be leaving [former company] on [date]. I will be taking up a new position at [new company].
My time here has been very valuable to my career. I have greatly appreciated the opportunity to work with you. I cannot begin to tell you how thankful I am for the support and advice you have given me during my time at [former company]. Although I will miss everyone here, I am very excited about this next step in my career. I know that the insight you have shared has prepared me for the new challenges that await.
Please keep in touch with me. You can reach me at [contact information]. Thank you again, for everything.