Most likely it's either the thermostate is stuck partially closed, or the cooling fan is not comming on... but check these items first:

1) Make sure the coolant level is full in resevoir... and in the radiator... afte the engine is cool, remove the cap... it shold be full.

2) Check the drive belts

3) Does the heater in the passenger compartment work??? IF not, stop what your doing and get it towed to a dealer... the water pump is probably not working properly.

4) Check the cooling fan. You should be able ot hear it come on or see it come on when the temp passes a little beyond the halfway mark. It also should come on when the A/C is turned on.

5) If the fan works in both cases, then the thermostat is probably sticking partially or fully closed. The thermostate regulates how much coolant flows to the radiator instead of being recirculated/bypassed back to the engine. THis bypass allows the engine to warm up faster and stay cool when the cooling effect of the radiator is not needed.