Buy a shotgun. A twelve gauge Mossberg will run you $150 to $250 at your local sporting goods store. Buy a box of five double ought buckshot for it along with two boxes of the heaviest birdshot the store offers. Before you leave the store have the clerk remove the wooden dowel from the magazine so you can load five rounds in the shotgun instead of just two.
You'll find the shotgun to be the best home defense weapon for someone who is unfamiliar with firearms. It's easy to use and the mere presence of a shotgun is often enough to scare away any attacker.