I am going to guilin in the mid or starting of december so what do you recomend for clothes?

The temperature may range from 45 F to 60 F during that period of time. a little bit cold. You are suggested to prepare jackets, sweaters and pants. Have fun!

The problem was just that i am staying there for a long period of time about 6 years cause i am going there for my graduation at medical university.....so if there are any other useful tips so please tell me.

Oh, it's really a long time. You are suggested to bring some warm clothes for December and leave more luggage space for other necessities. Upon arrival, you can go shopping for more clothes. Generally speaking, you can find many fashion clothes with acceptable prices. Believe me, China has lots to offer for you.

And one more question i cannot speak Chinese language ...does the people or shopkeepers can understand or speak english.

The local peole and shopkeepers can speak very simple English. There may be barrier in communication. You are advised to take some notes bearing Chinese characters, and show the locals and shopkeepers.