octopus card : where can I buy it at hongkong airport? How much to charge it (for 11 days) please?

After checking out, you can see a round service counter, where you can buy the card. For 11 days' stay, the Standard Card is suggested. At the very beginning, you need to deposit 150HKD: 50HKD refundable fee and 100HKD initital value. I have no idea which transportation you will use most, so it is hard to tell how much money you will spend. Anyway, I suggest you deposit 200-300HKD first. If not enough, go to the Customer Service Center of any MTR station to top up.