China uses the GSM standard for mobile phone communications. You can purchase a Chinese SIM card from one of the two main suppliers: China Mobile and China Unicom. If your current mobile phone is not compatible with the Chinese network, you can acquire a Chinese mobile phone relatively cheaply from one of these suppliers, from electrical shops, or by hunting for a bargain at one of the many mobile phone markets around the city.
China Mobile offer a pre-paid SIM card for $28, which you can recharge at shops throughout the country by purchasing recharge vouchers. They offer the option to set up your balance checking and recharge prompts in English, and have English speaking customer service staff.
China Unicom offer 'ru yi tong', a pre-paid, rechargeable SIM card.
Both of these services have the advantage of not requiring credit checks or monthly payments. The recharge vouchers, used to add credit to your account, are widely available in electronics shops and in news kiosks.