There are two basic types of NOS setups: the wet system and the dry system. The dry system incorporates the use of a spray nozzle that delivers NOS directly to the intake manifold. In this setup, the NOS is only added when the nitrous system is turned on. The wet system combines both the standard fuel and NOS together while feeding them into the intake manifold. Dry systems cost less, but require upgrading the fuel delivery system and injectors. The wet system costs more, but vehicle modifications are minimal. Because of this, the wet system would be the best way to go for most applications.
For more info, see How Nitrous Oxide Increases a Vehicle’s Performance