Well, strictly for home protection, it's hard to beat a nice 18" or 20" 12 gauge staggered with 00 buck and 3" slugs every other round. You can pick up a Mossberg 590 that will hold 9 rounds of 12 gauge, slap on a folding stock and heat shroud over the barrel, and if you're really frisky - maybe a nice flashlight on the foregrip - and THEN you have an absolutely GREAT home defense gun, plus a whole bunch of attention getting capability going for it too ... and you can pick one up for a lot cheaper than a decent semi-auto.

Personally, my vote would be for a 1911 (depending on the size of your hand at 15 years old.) You can get a .22LR conversion as mentioned above and shoot it as a .22.

Another suggestion would be for a Browning BuckMark in .22 to get a feel for the gun and get some experience shooting it.

That said, I noticed you wanted something with a high capacity magazine, and that's the point I really want to touch on. Please, don't focus too much on magazine capacity or on having 15 or 20 rounds in the gun and rely on that. It's not often you get attacked by an entire motorcycle gang.

Focus first and foremost on technique, grip, and shot placement. Then work some on speed reloading. You don't even have to shoot the gun, just go stand in front of a mirror if need be, hold the gun out like firing and drop the magazine, while slamming a new magazine into place. Work on this over and over and over. Work your reloading speed and your accuracy, and never look back or become dependant on high-capacity magazines.

If you go with a good 1911 (which I personally recommend VERY highly), then you'll be set for life. With the .22 conversion, you're all set for stuff like squirrels and rabbits, and a .45ACP is a truly excellent gun with a long history as a man-stopper for personal defense, and can make a good hunting gun as well, although there are better choices for hunting guns, namely almost any good revolver.

Whatever gun you go with, learn it inside and out. Practice shooting it, practice drawing it, practice reloads, double-taps, everything ... but practice with it, and rely on SHOT PLACEMENT instead of high capacity magazines and just spraying bullets "out thatta-way".