You should fly to Beijing Capital Airport firstly. When you arrive at the airport, please take the shuttle bus to Tanggu, Tianjin City. The bus terminal is close to the port so that you could hire a cab driving there easily.

Shuttle bus from Beijing Airport to Tanggu, Tianjin could be found at the end of this page: https://www.travelchinaguide.com/cityguides/beijing/transportation/airport-shuttle.htm

We are going to Beijing in March, but our flight Air Asia from Perth/Kuala Lumpur actually lands at Tianjin airport. So you can get a direct flight to Tianjin.
My query now is how to travel to Beijing from Tianjin airport using the bullet train. Some people say we have to go to Tianjin railway, some other said we can catch the bullet train at the airport???

Hi Mr. Tan, of course you couldn't take a bullet train at the airport. When you arrive at Tianjin Airport, you should take the shuttle bus or a cab to get to Tianjin Railway Station firstly and then take a train to Beijing.