A good start consists in crossing the line as soon as possible after the start. But it is not enough, you also have to start: that is to say with as much speed as possible.
You must observe the other competitors and the elements (wind, currents) and use them to your advantage.
Here is a video that explains how to position yourself for a starboard tack start.
In the video below, you can see the importance of starting with speed, when all the racers are grouped together on the line, they are all sailing in a shifty, turbulent wind. They are also much more at the mercy of possible tipping.