The turbo boost is an exelent way to get in front of people easily. Its also a very simple trick and there are many ways to do it.

Starting Line
This is the most important one if your kart or driver does not have good acceleration. Right when it says "GO" press (A). It is very hard to do it that way. An easyer way to do this goes like this. When its on "1" start tapping (A) rappidly. This will work 80% of the time.

Two Player
This gives you 2x the boost than in one player. When its on "1" start tapping (A) rappidly. This will work 65% of the time. The reason it will be a less percentage is becuase you have less of a chance that both players will press (A) at the right time. You can also try pressing (A) right when it says go, but that brings the percent down to 43.

Lakitu's Drop
This is when Lakitu picks you up becuase you fell off the track. Right when he drops you, hold down (A). This will give you a small boost, but it still saves time.