December

December is a warm month...because(cause sounds too informal) this month not only has a Christmas atmosphere and we can celebrate the festival(although Taiwan doesn't have vacations),but also has the last day of December...um...we'll welcome to 2011....
(I suggest breaking this into several sentences so that it will flow more naturally)

December is a cold month in Taiwan because almost people who wearing coat and boots walking on the street. (Because suggests a causal relationship, so unfortunately, the meaning of your original sentence would be: The reason that December is cold is because people wear coats and boots, if they didn't wear those things, than suddenly December would be warm, I suggest using the word 'so')
December is a cold month in Taiwan, so most people wear coats and boots while walking on the street.

For me, next year is an important year, because I plan to be a student again, and study new things such as foreign languages. I hope I can improve my English and learn more Spanish vocabulary. If I don't earn enough money, maybe I'll be a student the year after next year.

I want to ('wanna' might be spoken casually, but it is never written) become a External assignment personnel who can visit many countries... to experience different cultures and concepts.

Good luck!