1) Many US stock brokerage companies have branches in Hong Kong, such as Bear Stearns (which has been acquired and no longer exists), such as
Etrade, you can open an account in Hong Kong. They usually have the toll-free customer service number of China Tietong or Telecom.
2) US stocks should be bought in US dollars, and how to exchange foreign exchange should be consulted with brokers.
3) The U.S. government does not levy taxes on foreign investors, but you should check your identity every three years, which means that you should send them a W8 BEN form every three years, otherwise 28% of your income will be withheld by the IRS, and redemption will be very troublesome.