Hi. Travel from Beijing to Shanghai on a G train on Friday 10th October 2014. Do we need to book?

We hope to catch the rail at about 10am. Will all 1st Class seats be full if we buy our ticket when we get to the Beijing South Railway Station? We're wondering if we should ask our hotel to book them for us a couple of days earlier.
Whichever way we buy the tickets,what time should we arrive at the railway station? Thanks for your help.