Like the last chap said, under the bonnet.
It is usually somewhere near the power steering pump, which in turn is near the belts etc end of the engine if FWD usually to the left
colin, June 2009
Some look like this...
http://www.acs-pro.de/resources/Tuscan/H0169.jpg
The Power Steering Reservoir will be somewhere under the hood - much will depend on the specific car