Please let me know what do you want to read from the pin?
A digital signal or an analog voltage value?
The API macro reads a digital value on the pin. If you want to get a digital value on the P0_0 youcan use a notation like CyPins_ReadPin(CYREG_PRT0_PC0)
If you want to get an analogue value, you must put an AD converter on the CYSCH page and connect an analog pin to the ADC's input.
We are interested in reading an analog value from GPIO Pin P_0_0
There is a Code Example project "ADC_VoltageInp" You can download and install to your PSoC Creator.
Select a menu item "File->Code Example..." on the PSoC Creator.
Type "ADC_VoltageInp" to the "Filter by:" text box on the "Find Code Example" dialogue as follows.
Click the "Create Project" button to create a copy of the code example to your workspace.
Please refer the operation manual installed with the code example project.