here's a link to a formal template on the microsoft's website. You should be professional and it's preffered to give an official note (printed out letter) about your resignation - especially, those templates are professionally worded.
http://office.microsoft.com/en-us/templates/CT011224461033.aspx
Good luck.
Added:
It's also beneficial to have your resignation notice (worded by you, not your manager) to be attached to your file for the future reference.