What do I need for a gas blowback pistol?

The AEG I was saving up for is out of stock everywhere so I decided I'm going to get a gas blowback pistol. I found a couple that I like but they are Co2. I have no idea what i will need for this gun. I mean, obviously, I know I'll need the gun, BBs and a cartridge of Co2, but what else? An adapter?