the water of the radiator

it should be in the upper right hand corner with a yellow lid that says 'do not touch when hot'

Your owners manual will have good information on how to check and add fluids safely as well as information on the correct types of fluids to use. As a general rule for coolant you should check coolant at the overflow tank, there are max/min level markings or cold/hot level markings, the coolant level should be between these marks at all times.