start with the easiest first, check your coolant level and top it off if needed. then let your car idle until it comes up to operating temp and CAREFULLY check your coolant level again. if it is low you will have to examine all hoses and connections for leaks. after that the other possibilities are a bad thermostat or a radiator fan malfunction. describe the conditions at the time; stop and go traffic/ long periods of idling/ air conditioner on or off/ etc.