留學之前多久要去那兒

if it's possible, then go earlier ~ like around 1 month or even more than that...

becuase you can get used to the environment and at least know your way around the place before school starts

and you can also take the time to actually experience and travel around US a bit ...

cuz after school starts, you may not get enough time to actually enjoy staying at USA =) thus should take this chace to explore the place a bit

plus if you are not staying at school dorm., maybe you would like to go earlier to find an apartment? if so, then better go 4-6 weeks earlier

for other preparation, i think around 1-week is actually good enough

Good Luck ~

我觉得一般早两个星期到步买定生活必需品就可以啦.
您也可以问问有没有一些ORIENTATION的课程要上,有的话就可以早些过去又可以认识多些朋友.