How to get to Zhangjiajie

Hi, I would like to know the best secure, unexpensive and efficient way to get to ZJJ. My itinary is Beijing (the first city) - Xi'an - Shanghai - Hong kong (last city). I don't know from which city i should leave and on what transportation. Xi'an to ZJJ to Shanghai or Shanghai to ZJJ to Shanghai or Shanghai to ZJJ to Shanghai, or whatever... I'm little bit lost in my search.
Thank you in advance.