At the options screen, select "Simulation". Once you have gained your racing license and have a decent car, select the "Racing Calendar" option. Select the "Single Player" option, defeat your opponent, and you will gain $400. You will then be asked to either "Restart" or "Exit To Menu". Select "Restart", then just race, win, and repeat as many times as desired to get as much money as desired.
When fully staged, hold B + R, making sure you press B first. As you are holding both buttons, notice that your RPM is rising. Get your reaction time correct, then release B to get a big boost. Note: Do not worry if your car does a wheelie as it will not affect its speed.