First off, call it nitrous. NOS is a brand name. This isn't The Fast And The Furious.
You can hit an actual switch/button, or you can have a wide-open throttle switch where it kicks on instantly when the pedal is floored.
Most bottles are 10 pound bottles, but 15's are available. I haven't used nitrous on my Viper, but in my old Mustang, you get about seven 1/4 mile passes with a dry 150 shot in a 10 pound bottle without purging. I don't see the point in purging on a street car. I just "purge through the motor." Now, yeah, if you are track-only, it makes sense.
Best thing you can get is a bottle warmer. Remote openers are nice too.