Here's the basic idea: Use a general-purpose digital output from the Arduino, which is 0 or 5 volts. Connect it to a relay (Radio Shack sells these; for example
This One, but you should be able to find something locally. It's OK that it's 3 Volts, it's not that critical.)
The arduino controls the relay. The relay switches whatever voltage you want to the solenoid valve. Done.