there are not many things it could be, check the radiator if you can see inside it for restricted fins or feel it, it should be very hot at the upper hose and get a least very warm on the bottom. Any cold spots would indicate a restricted radiator.
Also make sure the radiator fan comes on, this is a more likely problem. If its does not check all your fuses first.
IF all that is OK When the car is stone cold, remove the radiator cap and start it up to see if will spit coolant out immediately after starting it. If it does that, you likely have a blown head gasket. Reply back if you still need help after checking these items..Glenn