There is counterfeit money everywhere, so no matter what anyone tells you always change money at a bank. It is better if you remember to carry small change with you, especially if you are taking a taxi. Before you leave your hotel, get 200 or 300rmb in small notes (5s, 10s, 20s and 50s) to pay for the taxis and small purchases on the streets. The driver will not have to give you back change larger than a 5rmb note.
If you are in Hong Kong, never change money in a hotel, as there are many exchange booths outside with great rates. In China, though, banks are the safest and have the best rates.
If you do change money in China then a quick conversion example would be: $1 = 6.2 RMB. For example, if you give $100 U.S. dollars to exchange, you will receive 620 RMB. Conversion rates change so check with banks for current rates of exchange.