If you plan on using public transport, consider getting an Octopus stored value card
If you plan on using public transport in Hong Kong, consider getting yourself an Octopus stored value card: it'll make your life much easier, and you can even use it in vending machines, convenience stores and other places (Where to use your Octopus).
These cost HK$150 - you can buy them in MTR and KCR stations, as well as Seven-Eleven stores and elsewhere. Of this money, HK$50 is a deposit, the rest is for your travels & purchases in Hong Kong. (As and when you're done with the card, take it to somewhere selling Octopus cards; you should get your deposit refunded, as well as any money remaining.)
You pay by holding the card near the yellow rectangle on an Octopus device, such as at station turnstiles or on buses - you'll hear a "beep" if it's accepted. (No need for the card to be naked here; you can keep it in your wallet or even your bag [if it's near the outside].)