Hi, I'd like to book a bus from Shanghai to Xi'an next month.

You can find the price, schedule, and book tickets at the official website of Shanghai General Long-distance Passenger Transport Station. But it only has Chinese version. You may ask help from your Chinese friend if there is any. As I know, there is one departure to your destination from the General Long-distance Passenger Transport Station at no. 1666, Zhongxing Road. The bus leaves at 11:00, and the fare is CNY 380.