Just like any system that runs coolant, it can be anything from a sticking thermostat, to a failed thermostat, to a clogged radiator, to a bad sensor, or even an air bubble in the system. Could be a blown head gasket or the fans aren't coming on.
Could be a whole mess of things. Would have to troubleshoot to pin point it. Time to get dirty. I would recommend getting a PDF file of the shop manual for your car if you plan on digging into this thing in the future. Would save you a LOT of wasted time on here waiting for an answer.
Also, use the search feature and google. Most of the stuff on these has been documented in very much length and there are even how to articles and videos on a lot of the maintenance and work that would need to be done.
