One possible version：
Dear Mr.Wang,
We haven’t seen each other for ages. How is everything going with you?
I am writing to tell you the good news that I have already been admitted into beijing university in advance。Actually speaking，you are the very person I should first say thanks to。It is you who helped develop my interest in English。Without your help，I would not be good at English。
Beijing university is a place which I have been Longing for。I am sure I will learn a lot there 。besides my major ，I will take part in all kindes of activities ，which will surely do good to my future career。
I know you are always busy with your teaching work。Do take care of yourself 。it is known to us all that health is the most important 。next time I come back I will visit you 。
Best wishes ！
Yours ，
Lihua
略