If you are deciding to go with a single hand to hold your pistol, there is nothing wrong with that. However, it does require you to have a steady hand to be able to shoot accurately each and every time.
The first thing to consider is safety. Always assume that your gun is loaded. This is true whether you are using a one-handed or two hand grip. Safety first!
Next, place your dominant hand around the grip of the pistol so that your thumb comes up along the side on the other side of the pistol. Your finger should comfortably fit around the trigger so that the center of your tip of your index finger is against the trigger itself.
You should feel your hand curve perfectly around the firearm. The grip should feel natural and comfortable. Always ensure that the webbing between your thumb and your index finger is far enough down the grip so that when the hammer cocks back it does not hit against that webbed area in your hand. This can cause a fairly serious burn or injury.
While the one handed grip is not ideal, there are some serious advantages to it. First off, it allows you to be able to position and fire your weapon much quicker. There is far less delay when you are not trying to position your other hand over the grip of the firearm to be able to position it to fire. When fractions of a second matter, that is important.