There are a posibilities for this. You may need to replace the thermosdat, the waterpump, the radiator, the hoses, or the cap. You may just need to add coolant. Make sure the engine is cold when you remove the cap.
It may be that you do not have enough water/coolant or that the tank is leaking. Check that the thermostat is working as this can also cause an overheating problem. If the water pump is broken, it may overheat the vehicle. Check that the radiator hoses aren't leaking. Test your radiator to see if that may be the problem.
First off, check the coolant reservoir, if it is empty, refill it. If it is full, it may need flushed if dirty, If that it not the problem, your thermostat may be stuck, it will need to be replaced.
it sounds like possibly one of two things. Either your fluid level is too low. which can be fixed very easily by adding more coolant or possible the thermostat is not functioning correctly. Which is not really tat expensive, but would require a visit to a shop to have fixed
