Dear ...
Please accept this letter as resignation of my position as __(name of position)__, effective __(date)__. Only after long and careful consideration of all factors did I finally make this tough decision to resign. I regret leaving friends here but I feel this change will be beneficial to my long-term career goals.
I am giving __(#)__ weeks notice, providing you with the opportunity to find a suitable replacement. I am more than willing to provide training and orientation to the new person. Please let me assure you that I will complete any outstanding projects prior to my departure.
It has been a pleasurable learning experience working as part of your team, here at __(Name of organization)__, and I wish all of you great success.
Sincerely,