May I getfirst class ticket on 4th September from Beijing to xiamen by bullet train and what is the price for two People.

Yes, you can.
The online ticket is put on sale 20 days in advance, so you can book with an online agency at least 20 days before your departure (this website accept early booking and you can book with it now).
You can also buy the ticket at railway station by yourself, and the tickets are available 18 days in advance at railway stations.
the first class seat for two people is CNY1,284 X 2 = CNY2,568