Im in the united states. How do I book a flight from Shanghai to Nanchang?

Hi, how luck you are 'cause I just booked a ticket from Shanghai to that city, so I suggest you book one in the following website:
https://www.travelchinaguide.com/china-flights/
It will be helpful to you.
Since you will arrive at 18:00, I suggest you go to Shanghai South Bus Station to take the long-distacne coach to Jiujiang. It departs at 20:30 and costs CNY 310. Hope you can catch it.