Been a lotta talk going around about this, up to and including Joe Biden (he likes shotguns). Long post on one of my favorite blogs, all about various calibers, stopping power, full of recommendations by name of obscure guns I barely even heard of. The kind of jargon that goes well in gun magazines. So, here are my recommendations for the total newbie (doesn't own a gun, hasn't fired and gun, doesn't watch many action adventure movies).
Get one that you can shoot well. A .22 caliber hit beats a .44 caliber miss. The bigger guns are more more likely to kill your target, if you can hit said target. Bigger means heavier, harder to hold steady, kicks harder and has a louder report, all of which add up to harder to shoot. Compromise on something you can shoot well, rather than a Dirty Harry style hand cannon.
Handguns are convenient, fit nicely into a drawer, a purse or a glove compartment. It is also VERY difficult to hit anything with a handgun, even at very short range. Long guns are much easier to aim and get hits with. They are also more powerful than handguns, a hit with a rifle or a shotgun is much more likely to kill your opponent than a hit with a handgun.
To do any good, you have to figure on doing some practice shooting. You need to practice long enough to keep all your shots inside a 10 inch circle. (At 25 yards with a handgun, at 100 yards with a rifle). Always wear ear defenders when shooting, they will improve your accuracy. The report of a gun is so loud it scares most of us, and the scare makes us jerk the trigger when we should be gently squeezing it. Shooters call this condition "flinching". Once a flinch is learned, it's hard to overcome. Ear defenders muffle the report enough to prevent a flinch from developing in the first place.
The fit of a hand gun to your hand is very important. The right fit prevents the grip from twisting or sliding in your hand as the gun is fired, which makes the second shot more likely to go where you want it to go. A regular sized handgun is easier to shoot. The little snub nose jobs are harder to aim (and grasp). You really have to shoot a handgun to know if you are going to like it.
Revolvers are more dependable than automatic pistols. Revolvers have no safeties to forget, need little lubrication and have no springs under compression waiting to break. Just pull the trigger and a revolver goes bang. Automatics not so good. American Rifleman magazine did a comparison shopping piece on small automatic pistols not long ago. For each gun reviewed, they listed the number of times it jammed while shooting it. Stick with a revolver.