Dear xxx,
Long time no see. How are you doing lately?
好久不见，最近还好吗？
I'm writing to tell you that I was admitted to a key university. It is my dream for many years, and finally my efforts paid off. I'm so happy that I'd like to share my happiness with you.
我写信是要告诉你我被一所重点大学录取了。这是我多年的梦想，最终我的努力得到了回报。我非常高兴，所以我想要和你分享我的快乐。
I know you still have one year to go before attending the college entrance examination. I'd like to give you some suggestions that may help you. In the first place, don't be panicked. Moreover, making good preparations is important to your suceess in the examination. Last, keep a normal heart.
我知道你还有一年就也要参加高考了。我想要给你一些也许能帮助你的建议。首先不要紧张；其次做好充分的准备对你考试的成功很重要；最后，要保持一颗平常心。
So, wish you good luck. I will be wating for your good news.
祝你好运，我也期待你的好消息。
Yours,
xxx
参考资料：英语牛人团