Any racer knows that upgrading a car is a key factor in a race. That is why; most of the money that they got from the prize money goes to upgrading their car. If they won't do that, then they just can't compete with others, who are taking the game seriously. Besides, the prize money is gettting bigger.
Drive your car as fast as you can till you finish the race. Try to grab the booster, so you can use the turbo. Each race will have different condition for you to race in the next level. You can unlock new cars by grabbing the third spot or better in certain races. After each race, you will get a portion of the prize money depending on the position you got. You can use the money to buy upgrades.