Depends entirely on the exact light you have.
Some lights like Crimson Trace's new one has a button on the front of the grip you press with the middle finger of your firing hand.
The Surefire rail mounted ones have switches you can hit with either your trigger finger or support hand thumb. I find the 2nd option to be easier to do, but it requires a 2 hand grip. For the most part I use a handheld light for primary illumination and consider the weapon mounted light to be a backup, for situations where my support hand is occupied with other tasks but I still need a light to make the shot.
So I practice running all my pistol mounted lights with just the firing hand.
I'm sure there are other designs out there, but those are the lights I've played with...
I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain.