I’m so glad you asked because there are a lot of choices and even more opinions about what's best (that's a whole topic on its own). To begin with, you have to sort out the differences between a revolver and semi-automatic pistol. It’s entirely a personal choice and the best way to be effective, no matter what you choose, is thru practice with the gun you use.
Let's Break it down:
Revolvers can be either single or double action. For personal protection, you definitely would want double action. Why? A single action revolver will require you to cock the hammer every time before firing...the hammer is the “sticking up part” at the back of the gun. Think old western movies and their gun fights. A double action only requires you cock the hammer once and then you can fire the entire bullet capacity without having to do anything else.
Pros: A revolver is super simple to use, rarely jams, and is easy to load.
Cons: holds only 1/2 the number of cartridges/bullets (called rounds) as an average semi auto and generally has a heavier trigger pull (you have to squeeze harder for the gun to fire- but in a stressful situation, that probably won’t be a super big deal because, well, adrenaline).
Semi automatic is what police carry and they can actually be single, double or single/double action, which is entirely up to you. Bullets here are in a “magazine” (ladies, it's NOT a "clip" - just forget you even know that word).
A single action in this style handgun will have a visible hammer that you have to manually cock before you can pull the trigger and fire the first shot. But you can continue firing once the hammer has been cocked.
A double action will have a slightly longer trigger pull because pulling the trigger does 2 things: cocks the hammer for the first shot and fires, then allows you to continue firing. You never have to manually cock the hammer here.
A double-single action allows you to either manually cock the hammer or, with a longer trigger pull, it will both rack the slide and cock the hammer for your first shot and again, allows you to continue firing after. Whereas, the single action has no hammer; but, once you rack the slide, chambering a bullet, you’re good to go.
Pros: Usually Holds at least 2x more bullets than a revolver with actual high capacity magazines available. Generally lower profile is easier to conceal when carrying.
Cons: More moving parts and more pieces in general means they are slightly more prone to jamming, tho semi automatic handguns have come a LONG way!!!
That’s a very high level overview though certainly there is much more to be said about this. Once you’ve selected your gun style, you need to choose a caliber, make, model, etc. That post will be coming soon; but, in the meantime, I hope this was helpful!!