There are a number of ticket options available for riding the Airport Express.
If you’re in Hong Kong for more than a couple of days the best advice is to get yourself an Octopus Cards. These pre-paid travel cards let you swipe in and out of most transport in the city. You can purchase a card from the manned ticket counter just in front of the Airport Express at the airport.
You can also buy single and return journey tickets, the later being cheaper than two individual single tickets.
There is also the Airport Express Travel Pass. This gives you a return ticket to the airport and three consecutive days of travel on the MTR. Good value if you plan of getting around the city a lot.