The easiest way to get to CityU from the Hong Kong International Airport is by taxi.
The fare will be about HK$300. (Travel from Hong Kong International Airport to City University of Hong Kong is about 35km and the journey time is about half an hour.
You may find more information on travelling by taxi
Alternatively, you may take the Airport Express and the MTR for less than HK$80 but you will have to change Lines three times. (The total journey time for taking the Airport Express and the MTR is about 45 minutes. Detailed information and the exact fare can be found here.) Ask for instructions at the information counter on the arrivals level.
For general information about Hong Kong, visit the Hong Kong Tourism Board.