getting out from Shanghai to Zhangjiajie

Hi,
thank you for all the information.
i intend to visit on October for 8 days. 4 days in Shanghai and 4 days (+/-) in Zhangjiajie (hiking in the park)
what is the easiest and fastest way to get there? should i buy in advanced tickets for transportation or i can arrange it easily during my stay in Shanghai?
thank you