If you're just going to have it for home defense, I'd go with a pump action shotgun. Either a Remington 870 or a Mossberg 500. Given your size, I would look at a youth model since the stock is going to fit you much better. You should be able to get something for around $300. Good thing is that you don't have to have great accuracy, just have to point it approximately at them and let 'er rip. With the right load, no one is going home after that....best case for them is that they are running for their lives.
If you don't like the idea of a long gun, get a revolver (in .357 / .38 special) since they are very simple to operate and reliable. They'll cost you more, but you can also take it to the range and have a little fun with it too.
There are pluses and minuses to any selection, but these are your two basic choices.