Pull over. If possible, pull over and turn off your car to allow the engine to cool. Open the hood to allow heat to disperse faster.
Do not open the cap on top of the radiator while the engine is hot. Doing so will release a high pressure combination of steam and radiator fluid that can cause very serious burns.
Check the coolant reservoir tank and fill if needed.
Determine whether to drive on or call for help. If the car was simply low on coolant and you were able to refill it, it may be safe to drive on. If the car appears to be out of coolant entirely, do not drive it.
Turn the air conditioning off. Air-conditioning puts a load on the engine that you want to avoid.
Use the heater to disperse engine heat. Flip the climate controls to vent, turn the heater all the way up, and turn the fan all the way up. If the weather is hot, the inside of the car will heat up quite a lot. Point the vents out the windows as much as possible to help reduce the temperature.
Keep a close eye on the temperature gauge or light. Pull over and turn off the engine if you have to. Remember that if your car overheats too much, it will ruin the engine.