To aquire more nitrous, try crashing things like running into other cars and hitting the walls, scenery, etc. You will get more nitrous for doing things like these. The bigger the damage you caused, the more nitrous you get.
You may find it helpful to save up your money early in the game. Why? The farther you get in the game, the more cars you are able to buy. These cars are much better and stronger than the earlier cars of which you had to choose from.
Create a new profile and use the following to enable the cheats:
GIVEALL: Unlocks everything
GIVECASH: Lots of dash