You may write a letter as follows:
I am happy to inform all of you that I am completing exact one year in this company on ________.
When I joined this company a year back, I was totally new in this company's culture and it was a hard time to match with it.
It was you all who had taken pains to teach me the culture and working style of this company. Because of your strong support and my Manager Mr./Ms. _______________ who had guided me time to time in my work, I could successfully complete this one year period in the company.
I am sure you all will continue your to provide your support and guidance, which will help me in performing the Best in near future.
Thanking you once again, with warm regards.
6th August 2012 From India, Pune