Arduino gps / gsm / gprs sim problem

Hello, I am just looking for some advice, I recently purchased: SIM808 GPS/GPRS/GSM Arduino Shield(Product Code : RB-Dfr-665) {https://www.dfrobot.com/product-1469.html}, but I have not been able to collect any GPS data? I am getting the "Open the GPS power success" in the serial terminal when I connect to an external power source, but nothing else is happening. In the AT commands it is coming up with AT+CPIN? which I can see means an error with the SIM card. (Just wondering if this is something to do with APN settings?)

Another query I am wondering about..I have purchased a GiffGaff SIM card that says it is 2G/3G/4G compatible. Should this be okay for the device? Also I am in the UK.

Any advice would be great for a newbie.

Lucy