Steam coming from under the hood may mean that the cooling system has overheated. Pull to the side of the road and turn off your engine immediately. It's a good precaution to carry extra coolant or water in your car to add to the radiator, but only after it cools down. Modern cars typically require a 50/50 mixture of coolant and water, so pouring only water into the radiator may lead to the engine overheating again eventually. If you only have water on hand, you can add it to the radiator, but make sure to also add coolant as soon as possible.