That is not a simple question and there is no simple answer. A personal firearm is a very personal item indeed. It has to fit in with your lifestyle, your clothing, your physical build, your local laws and its intended use. There is no one best answer for everyone.
My advice is to treat this as you would any major purchase. Do your homework. Research what's available. After you've done that, go to a shooting range that rents guns (probably an indoor range) and try out the ones you think might work for you to see what fits you best. Keep in mind that the fact that someone is standing behind the counter at a gun store does not necessarily mean they know anything about guns or their use. Some of them will be very knowledgeable and helpful. A few will cheerfully spout utter nonsense. All of them are salespersons. That's one reason you have to do your homework.
That said, here's some guidelines:
Stay away from off brands and cheap junk. If you're a beginner, buy new from a reputable dealer. A good quality firearm, properly maintained and cared for, will last a lifetime, so amortize the price over a few decades to reduce sticker shock. Remember, you're going to bet your life on this gun. The middle of a gun fight is a bad place to realize you should have spent the extra hundred dollars or so.
The minimum defensive caliber for semiautomatics is 9x19mm, aka 9mm Parabellum, aka 9mm Luger, aka 9mm NATO. (If you have a physical limitation that makes 9x19mm too much to handle, 9x17mm, aka .380, may have to do, but certainly nothing less.) The minimum for revolvers is .38 Special. Anything over .45 ACP for semiautos or .357 Magnum for revolvers will likely overpenetrate the target, wasting some of its energy and endangering who or what is beyond, so they're not recommended.
Note: Several companies are making remarkably small 9x19mm handguns these days. My wife loves her SIG-Sauer P239. If you can handle a fatter grip, Glock makes the subcompact G-26.
These days I recommend against revolvers for defensive purposes. Their capacity is too restricted and they're too slow to reload. It's true that revolvers have fewer stoppage modes than semiautos. However, most semiauto stoppages can be cleared in a matter of seconds with a little training. When a revolver jams, it's out of the fight and may require a gunsmith to get it going again.
When in doubt, you generally won't go far wrong with a Glock, but some people just don't like them. Nearly all ranges that rent guns rent Glocks, so it's easy to find out if you get along with them.
Tritium night sights are worth the extra expense. Most other bells and whistles aren't.
Re: Accessories:
Yes, you're going to need some.
You need eye and ear protection for practice at the range. A baseball style cap is also recommended.
You need a safe, secure place to store your gun when necessary. There are many options for quick access lock boxes and gun safes. (Caution: Guns left alone in safes have been known to breed and multiply.) Beware of trigger locks. They are accidents waiting to happen and do nothing to prevent theft. There are few things more embarrassing than going to your local police station to report a stolen gun. One of those things is coming home to find someone pointing your gun at you.
You need a cleaning kit. There are many on the market and they all pretty much work. Follow the instructions that come with them for gun cleaning and maintenance.
You'll need a holster. Sooner or later, there will come a time when you want to have your gun with you for a while. The middle of a riot or natural disaster is a bad time to go shopping for holsters.
A holster should be designed to fit your make and model of gun. Its mouth should be stiff enough to remain open when the gun is drawn in order to facilitate safe reholstering. It needs to hold the gun and itself firmly in place, so it will be where you expect it to be when you reach for it. Military style cover flaps are counterproductive. Thumb break straps will slow you down. A good quality concealed carry rig can cost $50-100. A simple Kydex or other plastic paddle holster will be much less, but won't be as stealthy.
For semiautos, you'll need a spare magazine carrier or two. The doubles are often uncomfortable to wear and some point one magazine in the wrong direction. For revolvers, you need speed loaders and carriers for them.
Above all, get training in safe gun handling and defensive use of firearms. That is absolutely critical. Without training, you're kidding yourself and endangering those around you. Don't let it slide.