How to get into main city centre from Hong Kong Airport?

Well, you can take Express train or shuttle bus to the city center.
By Express Train, you can arrive at city center within about 25min. To HK Station, it costs 100HKD and to Kowloon, it costs 90HKD.
By shuttle bus, it generall takes about an hour and costs 20-40HKD.

You don't need visa to HK.