[Igovert500]

You essentially have to rebuild your motor. It depends on your car and stock motor, some stock motors are built damn well, others suck. But if you are talking about a typical car, you will need to replace most of the internal parts in your motor.
As well, how big of a shot of nitrous(if you have already decided) are you trying to put in your car? Because that would make a big difference.
3rdly, nitrous is 100% street illegal, just FYI.
Lastly, may I suggest you do a bit more research on the car/modifications you intend to do. As you previously said, you don't know very much about cars. I suggest you research and decide if nitrous is really the route you wish to pursue. There are plenty of other ways to get hp and go fast. You might be interested in forced induction. ie. a turbo or supercharger. A supercharger for instance has the power 100% of the time, you don't need to refill any bottles, and it is legal. When you hit the gas, the power is there, you don't need to crack open any bottles or refill it every time you race. It just seems to me that you might benefit more from taking a step back and really learning/deciding what exactly you are going for, that way you will be more happy with the end result and have a more enjoyable time along the way.