I know we are trying to avoid a brand v brand discussion, so what I'm about to say is from what I've done before. So with that disclaimer out of the way....
As for leaning, one of the guns I started my wife out on was a
Ruger Mark II. It's an accurate .22 semi-auto pistol so the the ammunition is cheap. The slide is also quite easy to manipulate, and it's just fun to shoot. Later on, you can buy 'upgrades' and get much better accuracy out of it suitable for matches.
As for a smaller defensive pistol, one favorite is the
Walther PPK/S. It has a nice small size to it, and the .380 ACP round will definitely give you more stopping power than a .22. As for accuracy, at the range, both of us can get pretty good groupings with this weapon at 50 feet, so for defensive applications, this would work.
Okay, I'm done with the name dropping for now. Just echoing some previous posts, go to a local gun shop/range and just get the feel for a few guns. If possible, rent and use them. Something around the .38 and 9mm caliber range would be useful. Larger calibers start getting heavier and/or giving you less capacity in the gun. Small calibers might just tick off the assailant more depending on where the hit is. Barrell length is also something to think about - the longer the barrel, the more accurate the gun will be, but also be harder to conceal, and be heavier to carry, and vise versa. A barrel length of 3-4 inches should suffice.
In the end, decide what fits your hand size best, and what size you want, but don't let someone talk you into a weapon that will be the wrong size for what you want to do. After all, this is going to be your gun.