Ferry tickets to Zhongsan Port from HK airport

We (3 pax) will be arriving in HK on 15 Feb at 14:00 hrs.
We would like to take the ferry service directly from HK Airport to Zhongsan Port.
Please let me know -
1)which is the most convenient departure time of the ferry service for us
2) can we buy the ferry tickets upon our arrival at HK airport, if so, where is the location of the sales office
3) if we can buy the tickets online, which is the website for me to buy the tickets?
4) where do we board the ferry at HK airport?

Thank you.