Hi,moses.
Tickets for trains departing from Shanghai go on sale nine days before the departure day. You can go directly to the ticket office on the first floor of the Complex Ticket Building of Shanghai Railway Station to buy tickets. Travelers can also book tickets through internet. For further information, you could read this website:
https://www.travelchinaguide.com/cityguides/shanghai/transportation/train.htm
For what trains you should take, you can search on this website:
https://www.travelchinaguide.com/china-trains/
Highly informative and helpful. Good luck!