It could be a few different things. Your thermostat could be stuck closed, headgasket could be shot or cooling fan switch might be toast. If you are getting white smoke from the exhaust, the coolant reserve boils over after driving for a bit, this would lead me to think it is a headgasket issue. You can feel the two main coolant hoses form the radiator and if one is stone cold, and the other is scalding hot it is probably your thermostat.