For the technique of racing starts, see racing start.
A boat starts when, having been entirely on the pre-start side of the starting line at or after her starting signal, and having complied with rule 30.1 if it applies, any part of her hull, crew or equipment crosses the starting line in the direction of the first mark.

So if I cross the starting line too early, have I started? Edit

No! A boat that has received a rule 30.1 penalty after the preparatory signal cannot start until after she has discharged that penalty. However, such a boat is still racing until she either retires or finishes.