China has 3 major telecom operators:

China Mobile, China Unicom, and China Telecom.

You may go to a business office of any of the 3 telecom operators to apply for a SIM card.

Please bring your personal identification document (such as the passport) with some cash, and fill in an application form when apply for a SIM card. You may print out this sentence “我需要办理一张手机卡，我携带了护照和现金” in advance and hand it to the staff before they have found someone who speaks English to help you.

The staff will help you to select a service package according to your needs. Usually, a service plan with a monthly rate of 30~50 CNY will be adequate to cover your daily needs.

Once you have obtained the SIM card, you may use the following mobile apps for self-services to activate your phone card, check the phone bills, or top-up the deposit, etc.