Make sure iTunes is up to date. ... Once iTunes detects your '''iPhone''', it '''should''' ask you to go through a few steps to '''activate''' your '''iPhone'''. If that doesn't happen automatically, select your '''device''' from the left hand pane and you '''should''' see the option to '''Activate''' your '''iPhone'''
Your phone might be SIM locked. When you bought the phone and put a SIM card in was it the same carrier as the one you have put in now?
The first time you see this error, it’s better to wait. The chances are that the servers at Apple are truly busy or buggy which means the activation stuff isn’t going through. You should wait for a few minutes before trying again.