Glock, SIG, Beretta, HK, Springfield Armory, Browning, FN, Colt, Ruger, S&W, CZ, Walther.... There are a gazillion different makers who make good guns. I excluded Taurus and Kahr for a reason.
I would look at anything in 9x19mm or larger caliber for semi-autos and 38 Special for revolvers (327 Federal is brand new and is supposedly like 9mm...which is like hot 38 Special...nothing new, move along). If you stay with: 9x19mm, 38 Special, 357 Magnum, 40 S&W, 45 ACP, and 44 Magnum, you should never have any trouble finding ammunition. If you go with a different caliber, you'll have to shop a bit more and probably pay a premium. That's not to say 41 Magnum and 357 SIG are not good; they're just not common.
If you break it down even further, 38 Special, 9x19mm, 40 S&W, and 45 ACP are probably the most popular centerfire cartridges in the US at this time. Revolvers account for a small percentage of the market, but some people really like them.
I highly recommend that you go with a semi-automatic. Yes, revolvers are neat, but for self-defense, they have a few problems. Disregard any statistics (you'll only need an average of 3 shots) because the fight *is*...it is not what you want it to be. Some very common handguns:
Any 1911 in 45 ACP
Glock 17, 19, 26 in 9x19mm
Glock 22, 23, 27 in 40 S&W
Glock 21, 30 in 45 ACP
Browning Hi-Power in 9x19mm (really a classic suffering low sales) and 40 S&W
SIG P226, P228, P229 available in 9x19mm, 357 SIG (NOT 357 mag), 40 S&W
SIG P220 in 45 ACP
Beretta 92 or 96 in 9x19mm and 40 S&W
S&W M&P
HK USP and P2000 in various calibers
Springfield Armory XD (9x19mm, 40 S&W, 45 ACP)
Ruger: various models...sometimes feel goofy, but usually pretty good value. New SR9 is competitively priced
Walther: P99 in 9x19 and 40 S&W
Steyr: Friend swears by them, but not widely sold/supported in the US
CZ: Very good value, very reliable from friends' accounts. Give it a serious look.
Kahr: Small, compact and total ****. I've owned two that broke in <300 rounds, friends had several that broke...one had a serious break
Revolvers:
Ruger SP101 in 38 Spl, 357 Magnum, 327 Federal
Ruger GP100 in 357 Magnum
S&W -- dozens of models with 6, 7, or 8 shots of 357 Mag or 6x45 ACP
Taurus: crap, don't even bother
Charter Arms: probably crap, but could be good
Dan Wesson: Good, limited new guns but gun broker has used ones
***Most important part***
If you stick with the major manufacturers for non-1911 handguns, you should receive a reliable firearm that will last a long time. You won't know what you really want in a first gun. This is OK because there are so many features and price points that it's impossible to really know without doing lots of shooting. Some points to consider:
1) Fits the hand
2) Feels balanced
3) You can see the sights
4) Fingers can reach the magazine release and external safety (other controls optional)
5) Functions reliably (but you won't know that for awhile)
You won't know how to optimize for the purpose you have in mind. For a learning gun, it's just important that it works to a reasonable degree of precision. This does NOT mean tiny groups; it means a 4" circle on a human sized target. Target guns are good, but you mentioned double duty as a defensive firearm. Therefore, reliability dictates that the mechanics of the gun be a bit more loose than on a target gun. You won't really need adjustable sights, though some guns will have them (like a S&W revolver, even though it's a defense gun).
As mentioned previously, a good 22 caliber handgun makes a great LEARNING gun. They are typically less than $300. The Ruger MK II or MK III, Browning Buckmark, Beretta Neo, Ruger Bearcat, S&W 617, and similar are all very good. I've heard of and have seen problems with the SIG Mosquito and Walther P22s (just my opinion and experience).
Just do not mistake a 22 as a good defense gun. It's better than nothing, but not by much*.
* "...but not by much" glosses over a lot of info. The terminal ballistics of the 22LR are dismal. However, that doesn't stop people from dying due to 22 LR gun shot wounds. The difference is time. If you shoot the goblin, but he lives long enough to stab you, then it's not really a win for you if he dies three days later. The goal is to deliver lethal force in such a way that the attack stops as soon as possible. That does mean you have intent to kill (lethal force and all), but your motive for doing so is in reaction to an attack. Killing for personal gain is murder; killing to live is not murder.
Last note: get training. Pay for lessons. Don't waste years of your life trying to learn how to shoot handguns from gun magazines. Put them away and learn from a good coach.
. You can use light/low recoil .38spl loads(that cost less too) to gain your skills then move up to the .38spl +P, +P+ JHP rounds, then use the full power .357magnum rounds(if you dare!!!, 
). The Ruger SP-101 as a long and respected history in the US market. It weighs more than other .38/.357 snub revolvers out there but the strengh makes it stand up to rough use and +P/magnum loads.
) DA only hammer fired semi auto pistol. 
