Where do i get my octupus card if I arrive at 12 pm at the hongkong airport?

After landing, you may walk to the Arrival Hall in T1, where you can see a round service center. You can buy an Octopus Card there.

In my experience, it is best to go by shuttle bus. Take no. A21 and get off at Cameron Road, Nathan Road. Walk southwards for 2min and you can see the mansion. The bus shall take around an hour and cost 33HKD.