You need an asa fitting
Slightly worrying that you don't know what you need to attach the air if you're building a gun yourself
If assembling your own general design then no major worries
If you are building from scratch then proceed with caution.
A paintball compressed air cylinder outputs at anything up to 850psi (a co2 cylinder also outputs at 850psi which is the reason why paintball air regulators were set at that level)