Good question, first of all a camera, plenty of GB space and backup battery. You should also consider packing some pain relief pills for the stomach, food is great but can get tricky after a while. Everything is cheap so dont take too much cash, ATMs are around everywhere. Train tickets, in-country flights etc. Can be easily booked there.
Recommended spots: Xian (3 nights), the water towns (3 nights to cover at least three of them), Shanghai (minimum 4 nights), Beijing, you can be there for a week and not finish it! almost 6 years ago
Read a great book about the country. If you're into food, try Fuschia Dunlop's "Shark Fin and Sichuan Pepper." Into history, try a book by Jonathan Spence. Into the environment, try Elizabeth Economy.
Make sure to download some kind of phrasebook app or bring a good actual phrasebook. If you're going to need a taxi anywhere, like to your first hotel, make sure you have the address in Chinese characters!
There are lots of things to see. Sticking to big cities like Beijing and Shanghai will help you avoid culture shock (if you don't look Chinese, you'll run into a lot of pointing and staring). Click around on Wikitravel to see what looks the best to you. If you want to see lots of sides of China, try a week in a Western part (say Xinjiang or Tibet) a week in the South (HK, Sichuan, or Yunnan) and a week in a big coastal city (Beijing, Shanghai). almost 6 years ago