What train should I take to get from Guangzhou airport to Shenzhen North please?

You can take a bullet train from GZ Station directly to Shenzhen North, and the train is available from 6:00 to 22:30, which takes about 1h 30 minutes and CNY79 for a second class seat, CNY79.5 for a first one.
By the way, you can take Airport Express line 1 directly to the GZ Station, which takes 45 minutes and CNY18.