Where did you get the fire extinguisher idea from? I've never heard of that done before. You need a CO2 tank and a regulator. Many pre built regulators have the solenoid attached already.
I'm just getting my planted tank set up and dialed in, so I recently bought most of my parts. At the moment I have a 20 lbs CO2 tank, an Aquatek premium regulator (has built in solenoid) and a bubble counter. Also you will need a way to add the CO2 to the water, I built a reactor, though there are several items you can buy to do it for you, though they are typically more expensive and dont work as well.
That give you a good idea how to start?