Buy an Octopus card at the airport, put about $500 HKD is reasonable:

Arriving by Air – Octopus Hong Kong

From Airport to Hong Kong Island/ HKU: Take Airport Express Train to Hong Kong Station. It is about 24 mins.

Take the Island Line of MTR (Kennedy Town Direction).

To HKU station: Exit A2 for HKU campus

Note: please come at least 20 mins in advance because there may be a long line of waiting for the lift to A2 exit.