Answer for USA, Canada and countries running a 60 Hz power supply service. On a ceiling light outlet the same convention is used as on receptacles. White wire to the silver coloured screw and the black wire to the gold coloured screw. If it is a lamp socket that you are talking about the wire going to the lamp is called lamp cord. There is an identifier on the this cord, either a raised rib or a colour identified. The identified conductor is connected to the silver screw on the socket and the other wire connects to the brass coloured screw. It is mandatory that this convention be adhered to on table lamps as the "hot" connection is kept away from fingers as far as possible by placing it at the bottom of the socket. The shell in the socket that holds the bulb will then become the neutral. On the same note on table lamps if you replace the plug always make sure that the wide blade on the plug connects to the ribbed side of the lamp cord. <><><>
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.