Wi-Fi access is ubiquitous in China. They are often but not always free, most commonly in cafes and restaurants. Almost all hotels provide Wi-Fi access either free or at a cost, when booking your hotel, you should check the terms of internet access.
If you would like to access the internet from your phone you can purchase an SD card. You can do so at the airport or mobile phone shops if necessary, and you will need your passport to do so.
Surfing the Web from behind The Great Firewall:
The internet in China is censored. This means many popular websites, including Google, Facebook, Instagram, and Snapchat, are blocked. If you require access to these and any other blocked sites, you will need to sign up for a VPN (Virtual Private Network) before coming to China.