Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
I was happy to receive your letter notifying me that I have been accepted at [name of school] for the [spring/fall] semester of [year]. I plan to attend and succeed in every course offered in my program.
Please do not hesitate to contact me if you need any further information, or if there is anything I have missed, please contact me at [phone #/email].
Sincerely,