If you see steam from the engine compartment, you can turn off the engine and wait for it to cool down a bit before attempting to do anything under the hood. If there is no steam, you can open the hood and try to see if you can safely open the radiator cap. Only attempt to do this if you know how, for you can get seriously burned if there was steam or hot water under pressure to escape from a hastily removed radiator cap. Get a rag or disposable piece of clothing that would extend over the radiator cap and protect you from any rising steam or water. Twist the cap slowly paying attention to any steam which can indicate a dangerous pressure buildup in the system. If none is apparent, apply pressure on the cap while continuing to undo the cap slowly lifting the cap to release any pressure within the radiator. Once you get it opened, check to see if there is water or coolant in the radiator, adding accordingly if there is a lack thereof. Remember that you only add water to an overheating engine if it is still running, WHY?
If you turn the engine off, the engine’s various components begin to cool at different rates with the ones exposed to external air cooling quickly than those inside the engine. Killing the engine, stops the water pump from turning thus, preventing water from circulating through all the engine’s components equally to cool them properly. If you add water to the engine while it is off, the water cools the components it comes in contact first which can crack the engine block or other parts of the engine because of the different rates they are cooling. Hot metal expands, cool metals contract. Say the piston has cooled down due to less friction that occurred when you turned the engine off, the engine block which is more dense cools rapidly as you add water making it contract getting the piston stuck on the cylinder or bore resulting in a totally trashed engine.