Do a search - this has discussed many times.
In any case take the train. Buy a one day return (buy it at the departure train station or the arrival - you can go on the train without a ticket). 100 HK$ per person for the return ticket to Hong Kong Station (last stop).
Train leaves every 12 min, trip takes about 25 min.
Calculate around 60-80 min between touch down of the plane and arriving at Hong Kong Station, so you should be there around 18:00 pm.

If you are at the train station 90-100 min before your flight leaves you will have enough time for immigration, security etc. So be back there at 20:50.

Lots of things to do and see. In winter it will be too late for the sunset at the peak. Taxis are cheap. 20-50 HK$ for a ride to most of the attractive sites in the area.

Have fun! Hong Kong is a great city (you should really stay there for a few days....)