If the engine temperature starts to rise, stop the car, turn off the engine, allow the engine to cool* and then check the coolant level. If you cannot remedy the fault yourself you must take the car to a workshop.
If the engine temperature rises so high that the needle enters the red warning area, and you cannot remedy the fault yourself, the car must be towed to a workshop to avoid engine damage.
* If you do not allow the engine to cool before checking the coolant level you risk burning yourself from scalding water vapor when opening the filling cap.