Get a single barreled, single shot 12 gauge shotgun. Buy a used one. Use 00 buckshot.
As a hunter safety instructor I've spent a lot of time on the range with a lot of women and teenagers, and they all seem to find a pump action much easier to understand and handle safely than a break-open open-hammer single shot.
Great idea if you're in a dear blind, bad idea when more than one perps kicks in your front door. I'll admit, it's reliable as hell, but that's not a home defense weapon, but it's better than nothing.