You go to a rental agent, take yr pasport and international or local driverlicence with you, pay the fees and take care that you do not get a damaged car or with an empty tank.
Depending which city state you are they will have different rules. usually there is a local website to in form you. if they do not have an english page for these things, ask a chinese friend to help you out.
In wuhan you have to have local licence, but i drove car without one for several years. In Shanghai you need to do a local exam on the theoretical level what i understand. But it is quite easy to exchange your international licence. but you need be a resident. I hear beijing is the most liberal in this.

In some cities/provinces, the rental companies are open to accept the international driver license even though it is not quite legal. Many auto rental companies in Hainan provinces and other tourist cities are happy to let you take the rental cars but a deposit of 5,000 RMB is required in most places to ensure you have money to pay any traffic violation during your rental period. It is often cheaper to rent a car with a driver than to rent a car and drive yourself in China.

To rent a car and drive in China legally, you need a local Chinese driver licese. You may refer this article on how to get a local driver license:

How To: Get a Driver's License in China
http://www.echinacities.com/expat-corner/how-to-get-a-drivers-license-in...

Some airports in China such as Beijing Capital Airport now allow foreign travellers to get a temporary Chinese driver license if you have an internatinal driver license.