Have a GSM phone to bring to Shenzhen. where do I buy the SIM card?

You can use China Mobile or China Unicom's simcard. Since ou need to stay for a year, I'd suggest you subscribe a plan instead of a prepaid card. It would be much better if you tell where you are going to stay. Then I might find an office which is close to that place.

I personally don't suggest you buy the simcard at the airport. It's usually expensive there.