When playing in Grid Start race type, you have to hold down the Clutch button in the lower left corner of the screen to raise your RPM (revolutions per minute) level. As soon as the race starts and the countdown lights switch off, release the Clutch button!
Releasing the clutch at the right moment will give you a good start, or even a perfect one if the timing is spot on. Good and perfect starts give you more speed off the start line - helpful to overtake your opponents at the start of a race.
Please note that if you forget to tap on the Clutch button, your F1 car will never start. The same result happens if you keep holding the button and never release it.