Dear (Your Manager/Human Resources),

This letter shall serve as my official resignation from (Company Name).  While I am thankful for the opportunities I have received while employed with (Company Name) it is time for me to pursue other opportunities.

After careful consideration and debate, I have decided that the position I have taken will allow me to further grow my career.

I will be happy to assist (Company Name) in this transition to make it as smooth as possible.  My last day with (Company Name) will be (two weeks).  I appreciate all you have done for me.

Sincerely,