As Postdog indicates, the first thing to do (after putting some coolant in, of course!) is to look under the car to see if you can see coolant leaking away. If so, try to see where it's coming from. With a bit of luck it will be from a hose, or from where a hose is connected to some other part of the cooling system. (They're both very cheap to fix).
If the coolant is coming from the radiator, you can probably fix it (temporarily, at least) by getting some 'Radweld' from a garage or motor accessories shop. (You're meant to drain the coolant system before adding it but, in my experience, you can just put it in the radiator). A more permanent fix (i.e. by replacing the radiator) will be dearer than replacing a hose but still not wildly expensive.
However there's one possibility that could prove to be very costly:
Remove the oil filler cap and take a look at the surface of the oil. It should look exactly as you'd expect oil to do (i.e. liquid, with a shiny surface). If it looks 'like mayonnaise' the head gasket has almost certainly gone. Expect a bill of somewhere between £700 and £1000 to fix the problem.
Chris