If any gun is to be used for defense, it's most valuable feature will be reliability.
It has to go bangEVERY time you pull the trigger. Period.
Several manufactureres have been mentioned to satisfy that.

Next of importance for any gun is how it fits you.
It should be comfortable, and when you bring it up to eye level it should naturally be aligned with your eyes pretty well.

Minimum Calibers for defense:
Revolver=38
Semiauto=9mm(w/ fast hollowpoints, otherwise use a 40 or bigger)
Still use hollow points on the big stuff too.

Decide on a type. Revolver vs Semiauto.
Learnthe pros and cons of both before you decide.

Choose an action type:
Single, traditional double, double only, or striker-fired (like double only).

Other things to think about:
How important is ammo capacity?
How important is the ability to reload quickly?
Do you want night sights for defense?
(Even if you use a light to identify your target, you'll still need to be able to see your sights too.)
Do you want a manual safety? (Nice feature, but not necessary)
Do you want a magazine disconnect safety? (I don't)
Magazine disconnect safeties disable the gun if the magazine is removed from the gun. You cannot fire even if there is a round in the chamber and you have the safety off. Some people, mostly cops I think, like the feature because it can be usedin a defensive way to disable their own gun if a bad guy is wrestling it out of their hands. I personally don't want to get caught in the middle of a reload without being able to fire the bullet that is still in my gun.

Here's another tip to help you be prepared when things go really bad.
Learn as many different action types, and models of handguns as you can. Learn how the safties work, how to operate different cylinder latches, where the magzine release is, and what a loaded chamber indicator looks like. You may be able to make a move if the bad guy's gun is on safe or has an empty chamber.(risky, but out of the question if you seethe gun is ready to fire)Also, you never know when you may end up having to fight with someone else's gun in your hands. There won't time for a lesson, it'll probably be dark, and you'll be so pumped up on the moment that figuring out something new will be a real challenge.

Welcome to handgunning.