Hi Mathew on most cars it's the piston closest to the pulleys but just to check you could do this as sometimes manufacturers like to be awkward and put no 1 at the gearbox end of the engine. on the front pulley the larger one at the botom of the engine there should be a groove chiseled into it this is usually top dead center mark and there should be a mark on the timing belt cover or similar to line it up with, take off the distributor cap and see which contact it's rotor arm is pointing at then follow the sparkplug lead that's your no 1 if your car has no distibutor and has a coil pack you will have to get yourself a Haynes manual for your car to be sure. this info is not indigenous to the car mentioned and is a generalization of most motor cars, Hope this helps
Andy C, October 2009