Pull over! Many thousands of dollars are wasted on major engine repairs only because the driver "tried to make it" someplace while their car was overheating. When coolant is escaping from your radiator, your car's ability to stay cool goes with it. If your engine's insides get too hot, they start to distort, melt and break, leading to very costly repairs. Radiator repair is far less expensive than inner engine stuff.
If there is coolant gushing all over the place or you can see a broken or split radiator hose, you should try a radiator hose emergency repair patch or some duct tape. But if you have a pinhole leak, which usually appears in the radiator itself, you can save the day with a condiment. All you need is some pepper.
VERY IMPORTANT: Wait at least 15 minutes for your car to cool before attempting to make a roadside radiator or hose repair! Hot coolant will burn you!
Once things have cooled off, open the coolant filler cap and pour in as much pepper as you can find, up to a full shaker's worth. Start the car and let it warm up, allowing the pepper to circulate. With a little luck, the little pepper pieces will find the pinhole and clog it right up, giving you a chance to get to the shop for a real fix.
Don't fool yourself into thinking that the pepper fix is permanent. Not only is it unlikely to last long, you need to get all of that pepper out of your car's cooling system. It's not supposed to be there, and while it will not likely cause any damage, it's certainly not good for all those sensors and valves in there.