There have been some good suggestions so far. I'll throw out another idea. Consider starting with a .22LR rifle. Perhaps a Ruger 10/22. It's easy to use, it's not expensive, ammunition is very cheep and there's almost no recoil. I've found that the ladies I've taught to shoot are far less afraid of my .22 rifle than my .45 1911. If your wife is at all reluctant about firearms this may be a way to ease her into it.

Talking about using a firearm for home defense, where are you going to keep it so it is ready to use when you need it? If you have kids or there will ever kids in your house you need to keep it locked up. You can get a small push button lock box to hold a handgun that you can open in 5 seconds and keep it in the night stand. It's harder to keep a shotgun locked up and still have easy access to it.

If you haven't been there already checkout
www.calguns.net
Lots of good information there about shooting and the utterly ridiculous California laws.

If you're like the rest of us you'll start out with a couple of firearms. You'll have so much fun you'll want something different, so you'll buy another one or two or three. Then you'll need a safe to put them all in. Then a few years later you'll discover your latest purchases don't fit in the safe because it's full. Then you'll need a bigger safe. When you realize how much room it has you'll need to buy ... and it just continues.:D