Don't mess with the radiator cap or anything yet! Don't touch anything until you can do so comfortably. You could get a serious burn. You might have to wait as long as 45 minutes. As the engine cools, look for splits or tears in the hoses. The telltale signs of a blown hose will be coolant splashed over the engine and under the hood. This alone doesn't mean you have a blown hose, however. It could be a faulty thermostat that caused the radiator cap to release. If the cap has a pressure-release flap that's released, your engine may have only overheated, but if the cap is intact and you can see a leak in the heater hose, you'll be able to fix it. Once the engine is cool, remove the radiator cap. Put the cap back on after the fix.
Make sure the hose is slipped onto the coupling and tighten both clamps. Don't cowboy the hoses; they may be fragile and in need of replacement. Add water to your reservoir or radiator, or drive (no longer than 10 minutes!) to the nearest place you can get coolant. This fix isn't permanent, so make an appointment to have your hoses and coolant checked.
You're driving along running just a bit late for an appointment when steam starts belching up through the edges of the hood. Your car has just blown a heater hose and the coolant is vaporizing as it drains onto the hot engine. Well, we can't make you feel any better about what just happened, but we can help you get back on the road in a reasonable amount of time. If your car is more than 6 years old, no doubt your radiator and heater hoses have seen better days. Next time you're in the auto supply store, pick up a kit containing a heater hose coupling like the one shown and a pair of clamps. The kit doesn't cost much and is a good insurance policy. Just be sure you also carry a flashlight, a pocketknife and a set of screwdrivers (essential tools to always have on board).