I restored my iPhone but it always says Activation Error, what should I do?
Activation through iTunes or Over the Air (OTA) is when the device checks to determine if it is factory or carrier unlocked or if it needs a SIM to be activated.

It's almost certain that your failure to activate is due to not having a SIM card from the proper carrier. You do not need to have an active contract or phone number associated with the SIM (I have a 3GS which has an old SIM without a plan), it just has to be for the correct carrier.