Experience is usually required for CO2 regulators. You need to have a CGA320 nut and nipple for the high pressure side. This is standard in North America. Asia and Europe use different standards.
For the output side, it is usually 1/4" female NPT.
From there, it depends on the other parts you have (i.e. the solenoids I use are 1/8" female NPT, so I need to use a reducer from 1/4" down to 1/8", etc). These parts can usually be found at your local hardware store.
Other considerations when choosing a regulator:
Dual stage or single stage?
One gauge or two?
Chrome plated, brass or stainless steel (all work for our purposes; this is more aesthetics)
Maximum delivery pressure?
And of course,
uhhhhh I'll just let people give me links lmao, way to much to take into consideration and NONE of that information is on ebay