Complete as many races as you can. For each race you will need to complete it in a certain amount of time in order to unlock the next level. Beware of running into vehicles on the side of the road. It will force you to start over from a certain point in the game. You do get NOS to use when it fills up but you might not want to use it when going around big curves. Try to go around other vehicles on the road because they will slow you down. You will win money for completing a race and can use it to upgrade your vehicle.