放棄香港大學degree 移民到美國升學

I would strongly disagree with doing so. A Bch degree is a Bch degree, if you only need one more year to get it you should keep going until you graduate (unless your performance is so poor that you cannot graduate). Having a Bch degree opens so many more opportunities, you can get into another U programe directly, or you may even be considered for a Master degree program if you have good TOEFL and GRE scores. But anyway, I'll go ahead and answer you question:

1. 要不要先考TOFEL呢? 收費如何? 考試地點?? 資格?
TOEFL is only required if you are (i) non-US citizen, and (ii) not from a country with English as primary language. If you fit both the first one, you should take TOEFL. The cost is US$170 per test, and the center is in Sun Po Kong. For more information, go here:
http://www.ets.org/bin/getprogram.cgi?urlSource=toefl&newRegURL=&test=TOEFL&greClosed=new&greClosedCountry=China&browserType=Firefox&toeflType=&redirect=&t_country1=group_HongKong&date_selected_old=

2. 兩年前小弟alevel成績: 1C1D UE:D 可不可以以香港高考成績去報考美國大學呢
That I am not sure of. But you can ask the university.

3. 是不是要到美國從新來過, 到社區大學讀兩年後才駁上美國大學??
Mostly depends on your English competency. You can get into most community colleges and graduate with an associate degree, then you may be able to apply for other university (depends on your interest and financial abundance) and get into the third year. (US has 4-year system). Notice that some community college would put you to an Eng placement test first and consider if you can start the degree program right away, if you performed too poorly, they may require you to study ESL (English as Second Language) program first. Here is a list of most community colleges in the US, check with them:

http://www.utexas.edu/world/comcol/state/

Again, it's doable but I will never recommend it. Especially you are only one year away from graduation. Consider stay in HK for another year and get the degree first, that may put you in a more comfortable situation.

１．要考
你可以去到美國先考，美國既TOFEL個個月都有得考
無特別資格，你報名就可以考，費用同香港差唔多

２＆３．以你高考成績報美國大學報唔到比較好既大學，而且亦浪費咗你poly既２年，
其實你可以以你大學既成績transfer去美國既大學，應該起碼可以讀第２年，咁當然亦都視乎你既成績係點，如果你既成績唔係咁好，而你又想去好ｄ既大學，你咪報colleg，你poly既成績，應該可以transfer到第２年
無論點，你都唔需要再讀４年

1. You dont have to if you're a citizen or a Green Card holder. If you're 移民 to United States then I dont think you have to.

2. It's ok.

3. They might need you to. Because they think that you would have more time to get used to the studying enviroment if you go back to a lower form and start. everyone has a 4-year university life there. You will have the first graduation after you finished the first 2 years in 社區大學, then u get an associate degree. When you finished the last 2 years u get a bachelor degree. But they might let u just study 2 years and get the bachelor degree, it depends on the school rules.