In order to initiate Turbo Mode, you must reach first reach top speed. Keep accelerating until you reach 280KMH, you will now notice that your speedometer is flashing red, press B or the RB button to enter Turbo Mode. You will know you are in Turbo Mode when you see small bursts of flame coming out of the rear of your bike.
This Achievement is best accomplished on the early stages of the Beginner course were there are lots of long straights.