Family Name or Given Name: How to Address a Chinese Person
Strictly speaking, people in China have no first names. They have given names, which come after family names. So, one's family name comes first, followed by the person's given name - that's the order of a full Chinese name. If a Chinese tells you that his name is Wang Chunsheng, Wang is his family and Chunsheng his given name. In the West, some Chinese do follow the Western naming convention and reverse the order of their family name and given name, a fact that often causes confusion. While you travel in China, however, you can mostly expect people to present themselves in the original order of their name. If you're not sure, simply ask.
In China, as in the West, you can address someone respectfully by placing "Mr." or "Ms." before the person's family. You can be courteous by calling Wang Chunsheng "Mr. Wang". Similarly, as in the West, if a Chinese address another person by the person's given name, this indicates familiarity and friendliness. So, if you have got to know Wang Chunsheng pretty well, you can call him "Chunsheng".
There is a way for Chinese to use someone's family name and still show informality - to place the Chinese words "Lao" or "Xiao" before the person's surname. "Lao" means "old" and "Xiao" means "little", so the choice depends on the relative age difference between the two persons involved in the communication. Suppose Wang Chunsheng is in his 30s. A friend in his 20s (named Zhang Hongjun) may call his older friend "Lao Wang" or "Old Wang". In return, Wang Chunsheng may call his younger friend "Xiao Zhang". The word "old" in China does not have the negative connotation that it has in the West since traditionally Chinese culture emphasis seniority - older people are considered experienced and thus worthy of respect.
In this connection, the Chinese do not normally address an older person - old in the absolute sense - by the person first name. Suppose Wang Chunsheng is his 50s, people would not call him "Chunsheng". They'd call him "Mr. Wang" or "Old Wang".
To sum up, to show respect to someone, address the person with his family and call him "Mr. So" or "Old So". To be informal and friendly, greet the person with his given name or use his family name with the addition of Lao or Xiao - for example, "Lao Wang" or "Xiao Wang".