From Shanghai to Xian. I want to visit Terracotta Warriors.

Actually, you can choose to Xian from Shanghai either by train or by air. The train takes about 15-16 h for the whole journey and the plane only takes about 2.5 h.
If you want to reach all the major attractions in Xian, two to three days can be best. Terracotta Warriors, Huaqing Hot Spring, Big Wild Goose Pagoda and some others cannot be missed.
By the way, if you want to book a tarin ticket or the air ticket, the website may help:
https://www.travelchinaguide.com/china-trains/