Write a polite, mature letter directly to your current supervisor (not human resources).

Get right to the point.  You do not need to mention the reason you are leaving.  In this letter do not express any negative experiences that you may have had at the company.

If you feel obligated to mention a reason, just state that the new position is better suited to your personal and professional goals.

Indicate your last day of employment and offer to cooperate fully during your notice.  It is common business practice to offer a two week notice.

Thank your employer for the opportunities they have afforded you

Keep any conversations positive while you are working your notice.