This is a generic question and of course it is impossible to cover everything in one post , but there are a few important things to keep in mind when getting ready for such a great experience.
Having had the opportunity to visit different cities, to travel by train, car, bus and planes around the country we feel capable of giving a good orientation to the country. China is certainly a fantastic country that has witnessed tremendous development in the last decades. One of the things that still surprises us is how misinformed people are in the West about China. The most important thing is to be ready for the experience and to know what to expect.
Language: for any westerner this can be a real barrier. There are certainly many people who speak English in China, but not as much as in other regions of the world. The best thing to prevent problems is to approach the concierge of the hotel and ask them to write in Chinese a series of instructions or questions one may need such as “take me to my hotel” or “I need a Train ticket to Beijing”. I never leave my hotel without a detailed explanation in Chinese for a taxi driver in case I get lost.
Expect the best! China is a country with a tremendously good tourist infrastructure. Some of the best hotels of the world are located here, and one will find a good place to stay in almost any city. Just do not raise your expectations beyond the norm. Some travelers become demanding and unpleasant just with the excuse of “I thought tourist services here were amazing” in China one will get more with a “please” and a smile than by yelling.
Be aware of local custom and traditions. When one is the visitor one is expected to know how to behave in the place. These customs and traditions can be very different even inside the country or within the same region. For example something so simple as throwing a piece of paper to someone can be very offensive in China where one is expected to put documents or business cards in the hand of the other person. Be aware of food: It is quite different! And most of the times it is not even close to the version of “Chinese food” one eats in the restaurant close by home. Anyway there are many western restaurants in the main cities.
Do your research: Print maps, study them, become familiari with directions, addresses, streets and landmarks. Cities in China are huge, and sometimes once one gets out of the metro everything seems confusing, so use tools such as Google maps, remember that in the street few people will be able to understand your English, and as I said carry always your piece of paper with the address in Chinese, people are always willing to help even by signs.
Do not panic with crowds, some will complain that Chinese metropolitan centers can be overwhelming, this can be true in some extent. Public transportation works in a very good way, but the rush hour can be very crowded so be ready to go with the flow.
Know what you can do: One will be surprised how easy it is to move from one point to another not only in the same city, but around the country, with the exception of Tibet there are no restrictions to visit any region, one can wander almost freely the entire country, and even when the visa was issued for one city it is possible to go anywhere as long as one checks -in at a hotel or hostel in the destination. One will find almost no restrictions, it is true that YouTube, Facebook and Twitter do not work there, but anything else is almost the same. It is good to keep in mind not to take pictures of military infrastructure or government buildings and if one will take a direct picture of a person it is always polite to ask permission.