My .02. If it's for the house, go to wal-mart and get a remington or mossberg shotgun. Easy to hit what you're shooting at no matter how sleepy you are at the time. Point and shoot, and the action should be intimidation enough to persuade someone they can go elsewhere for someone easier. Relatively inexpensive. and they are good to go.
If it's a handgun...wait till you get enough money to buy something that shouldn't jam when you shoot it. If you're new to guns and don't think you will put in the time to clean it regularly, get a revolver...pretty low maintenance, and you ain't gotta worry about it jamming. If you think you will disassemble, clean, and reassemble fairly regulalry, go for the auto if you prefer.
Spend a little time and look at the pro's and con's of the calibers. 9's will have a high capacity...but those bullets don't just stop very easily, so if you did have to shoot in your house, you would have to keep in mind your family may be behind some of those walls. 45 flies a lot slower, so less to worry about that, but it has less capacity...40 is somewhere inbetween...etc. And if you do get one, go shoot it...don't just put it up. All that crap and more.
Take care.