From airport to Tiananmen square bus

Since the earliest available shuttle bus departs at 7:00 and the airport express departs around 6:30. You are suggested to take a taxi to Tian’anmen Square and then walk north directly to Forbidden City. The taxi takes about 50 minutes and CNY90-100.
Note: Online reservation for Forbidden City is a must, because they don't sell ticket on the spot now.

When back to the airport, you can take metro line 1 at Tian’anmen West, and then transfer to line 2 at Jianguomen. Get off at Dongzhimen and take Airport Express to the airport. It takes about 50 minutes and CNY29.

Where can i book the online ticket ?
And what is the route for the subway train from airport to tiannamen if i want to take the earliest one at 0630 ?

You can search gugong to find the official website for booking.

You need to take Airport Express and transfer to subway line 2 at Dongzhimen, then do another transfer to line 1 at Jianguomen. Get off at Tian’anmen West.