You'll need a 9V power supply/source and a [u]driver[/u].
What's the current rating (Amps or mA) on the solenoid?
An Arduino I/O pin can only supply 40mA maximum and the on-board regulator can only supply a few hundred mA.
Your DC-DC might work (along with a driver) if your 5V power supply can supply the required current.
If you use a step-up DC-DC converter, you are stepping-up the voltage and stepping-down the current. For example, if the valve requires 1 Amp at 9V, and the DC-DC converter is 100% efficient (which it isn't) you'd need (9/5) x 1A =1.8 Amps from the 5V supply. (2A is a more realistic estimate, accounting for some inefficiency.)