train tickets for China trip

Dear Mr/Ms

This summer i will travel china, I will arrive in china on the 19th of august and i will leave china on the 5th of september. i will start in Shanghai, then i will go to Zhangjiajie, after that i will go to leshan and after that to Xian and Beijing. I would like to do all of this by train. I have some questions:
- Can i order the tickets already, or do need to do this in less than 20 days before i will take that train?
- Can i pick up all the tickets at once at Shanghai Pu Dong airport?
- Can i pay by "ideal"?