You have already received some wonderful information from members here on your thread. So I will try to give you some information about selecting and purchasing your gun. As 45acpForMe said, do visit the Selecting a Handgun forum and read through it.
Take the time to visit a major gun show in our area. The Nation's Gun Show's next appearance at the Dulles Expo Center will be October 1st, 2nd, and 3rd. This is a very large show and there is much to see there. Another show to visit is the Fredericksburg Expo Center Gun Show which I think will be around the last weekend of September.
Guns shows offer an excellent opportunity to see and handle a wide variety of firearms. I strongly suggest that you do a lot of research (reading, asking questions, viewing videos, and even visiting gun shops) in advance of attending a gun show because large shows can be a little overwhelming for people new to all of this. You want to pay attention to terminology, features, and the pros and cons of particular guns as to how they will best serve your wants and needs for carrying. You'd be amazed at how involved this can be - trying to find and nail down that "perfect" gun to carry for defense is not a simple task. But don't get frustrated. The more you learn, the better off you're going to be.
Once you have developed a list of candidate guns to consider buying, start paying more attention to them: how do they fit your specific situations, how they feel in your hand, do they have the features you are looking for, and are they quality pieces.
Next up is to try to find a range that has examples of these guns that you can rent and shoot on their range. A friend who has copies of some of your choices and who is willing to take you to a range is a very good help as well. You want to see how the gun and you perform together. Can you hit your target consistently? Do the sights line up naturally or do you find yourself having to adjust your hold each time you aim the gun? Does it feel natural and comfortable in your hand? Can you deliver follow-up shots quickly and effectively? A lot more questions need to be answered but these are a good start.
Do not scrimp and go cheap when selecting a defensive sidearm. This doesn't mean you can't get a quality gun for a reasonable price. But don't think cheap. Other considerations are ease of maintenance, availability of parts and accessories, and comfort. Carrying a gun all day can become a problem if the gun is too heavy or too large.
Learn, learn, learn. And while you're learning, also learn to recognize and avoid the BS because God knows, there is one heck of a lot of BS in the gun world. You'd be amazed at the things people believe and pass on as fact which is actually completely and totally crap. Learn to recognize the crap and be able to separate it from fact.
To start you off on your quest, you should first decide upon what type of sidearm you want to buy: a revolver or a pistol (a pistol being a semi-automatic handgun in this discussion). From there, you can bet there is a huge assortment of guns from which to select and turn into your carry piece. I cannot stress quality enough. Regardless of whether it is to be a revolver or a semi-auto, think quality. If the gun doesn't go bang when it must, then everything else is a moot point.
I hope this helps you get going on your search. Please feel free to ply us with your questions.