If you buy and hold on btc/eth, you can use the atm machine or buy via some webpage.
If you want to actively trade, you need to open an account in exchange. There are lot of exchanges, famous one like bitstamp, kraken etc. Binance / bittrex is almost famous but they only accept cryptoccurrency transfer
After opening an account, you can TT the $ to those exchange.
There are several exchange in HK but bid/ask is wider.
Good luck
Again: Please be reminded that the market volatility is very high. Ppl can double the investment in day which also implies they can lose all in day. Also, i will recommend you to do more research on coins before buying. There are over 100 coins in the mkt and some of them may be scam.
Finally, I will recommend to use amount of money that you are comfortable even total lose. Like the amount of money used to buy penny stock...
you are welcome.
Again, the crypto market is very very volatile. Much more volatile than stock market. So use spare money, no margin, buy at dip, sell to make zero cost when the price is double or triple. And understand the function of the coin, news behind before buying.