In most instances of a pistol or revolver, the grip with the primary shooting hand (usually right hand for righties and left hand for lefties, also known as the strong hand) is the grip that makes most of the difference as to how you will shoot; and gripping a pistol or revolver one handed is the first thing you should learn about proper grip. The reason for this is simple: Unless you own some far fetched super macho two hand grip pistol or revolver, you virtually always draw the pistol, and begin to bring it into action, with one hand. Yes this one hand is often met by your other hand to maintain a proper two handed grip, but the truth is your for all practical purposes draw and first bring the weapon to play with one hand. Therefore I believe, it is extremely important to learn how to grip the firearms properly with that hand, before you even think of using a two hand hold. So how do you fill your hand properly? Well I guess I can best explain it by showing it to you. I'll show you how to make a one hand grip of a semi-automatic pistol.
Note that I am showing and explaining how to grip a pistol. I am not explaining how to actually draw from any given holster. Before you can draw from a holster you must first undo any retention device that keeps the pistol in the holster, and that is usually done just before you start step one or as you actually start to bring the pistol from the holster - all dependent upon holster design. Again this is just showing how to get a good grip.
1) When you get a grip for shooting the web of your drawing hand should meet the pistol on the backstrap directly underneath the tang. Your hand is going at the pistol grip almost like it is going to shake someone else's hand in that regard. Note the position of the thumb, and of the three fingers (middle, ring and pinkie) of the drawing hand. They are all poised and about to wrap around the grip of the pistol with a firm hold. Of course, you cannot see almost any of the index or trigger finger - but it is in contact with the pistol. You will be able to better see it in the next picture.
1-B) The view of the other side of the pistol shows the position of the trigger finger when drawing or first applying a grip to the pistol.
Note that when you pick up, draw, or otherwise handle a pistol (or any firearm) you leave your finger off of the trigger until you are actually ready to fire. If you draw with your finger already engaging the trigger, there is an increased probability that you will fire the weapon while drawing, and quite possibly shoot yourself. I learned this lesson when a coworker of mine did just that, he shot himself in his leg.
2) The next step is to assume a good grip on the pistol grip. Wrap your thumb and fingers, excluding the trigger finger, around the grip pretty much as shown. Note this the third picture, but it is marked as step 2 because it is the second action you take in gripping the pistol. The trigger finger remains where it was in pic 1-B, until ready to actually fire the weapon.
Have you paid attention to the position of the wrist? It is important to have the wrist in a firm straight position, basically in parallel line with the slide and barrel.
3) Once ready to fire the weapon, the trigger finger comes into position on the trigger, all the while as the other fingers keep firm position on the pistol grip. The trigger finger comes into contact with the trigger only as far as the finger tip pad. This allows for proper trigger control when you fire.
Note that the rest of the grip has, in essence, not changed from picture 2 above. The wrist is also still in the same position. You are now ready to fire and the pistol should already be pointing if not precisely aimed in at your intended target.
All it would take to fire the pictured pistol (a Beretta 92FS, 9mm, semi-automatic pistol) would be to squeeze (or pull) the trigger with your trigger finger in that position if it was loaded (and remember to treat each pistol you pick up or handle as if it were loaded, too many accidents happen because people thought the gun was not loaded, and firearms accidents are fatal all to often). If you have learned how to point shoot or aim in, you would likely hit your target. If you maintain that grip after each shot is fired, you would then be ready to fire again, and again, and again as needed. Once you have decided to stop shooting (at the range, in a self defense situation, in the field, wherever) you immediately remove your finger from the trigger. If you are holstering or putting the weapon down or away, you basically reverse the grip steps above. Of course, depending on the situation you want to make sure the weapon is safe. On this one you would be required to at least operate the 'hammer drop' aka: safety; or if you want to make sure it is empty, you would want to remove all possible remaining ammunition by removing the magazine, then unloading any ammunition remaining in the chamber, then visually and physically checking to make sure it is empty - all the while with finger off of the trigger and the pistol pointed in a safe direction.
Two quick pics of what you do not want to do when gripping a pistol follow:
You do not want to grip the pistol like this. Note the web of the hand is not properly positioned directly under the tang. This makes for piss poor shooting on the part of most shooters, especially new shooters, and or those not so good at it even after years of practice. This allows for the pistol to move around in your grip too much with the recoil from each shot. Because your hand is not in proper position, it also allows for the proper wrist position to be broken, resulting in the much feared 'limp wristing' and this can result not only in poor accuracy, but in pistol malfunctions, or so said all of my instructors over all the years I have been shooting.
The other no-no that I want to show you is in the last grip picture. It shows the trigger finger extended too far over the trigger, so that the first joint (or last joint depending on how you look at it) is actually completely over the trigger. This is not good as it can cause accuracy problems.
Of course, you probably will at times see someone holding a pistol improperly and still hitting the target. Yet I would wonder how much better they might do it they held it correctly. So, why pick up bad habits right from the get go? Start off with good habits, learn how to get it right from the beginning, and you will see that learning to shoot is probably much easier that you thought it would be. If you already exhibit some bad habits, why not get rid of them, use proper technique, and see if your shooting skills improve.
Don't depend just on this to teach you how to do it. Get instruction from a certified firearms instructor if possible, or at least from someone proficient in pistol shooting before you try it yourself.
All the best,