As Paddy said, top it up and if it looks like you are losing more water after this then you can try some rad sealer stuff. It could be the head or the rad, or some people have mentioned a problem to do with something concerning the EGR valve, but some rad gunk should help.
If you're only topping up a little bit then just water will be fine. If it's regularly needing topping up and you're adding a lot then you'd need to add coolant/antifreeze too otherwise the mixture will get diluted.