Visa
First you need to check if you need a visa to visit China as a tourist. If you need a visa, you need to plan ahead to find out how to apply, where to apply, how long it takes, how much it costs and etc. Those information can normally be found on the Chinese Embassy's websites.
Vaccinations
Then you also need to check if you need to get some vaccinations. The type of vaccinations needed depends on where you are from, your health conditions and medical history, so the best thing is to check with your health care provider 4-6 weeks before the trip.
Local currency
Get some Chinese currency and familiar yourself with the facial value of each type of notes. Prepare to haggle when shopping in China.
Learn Chinese
It's also a good idea to learn some simple Chinese before the trip, which you can learn via atozchinese.com
Food&Drink
Do not drink tap water and always drink bottled water or boiled water. Most food are served hot in China, so there is less risk to food poisoning