One firearm you may want to consider is the Kel-Tec Sub 2000. It is a small, foldable pistol caliber (9mm *yawn* or .40 S&W) carbine. Won't over penetrate walls the way a rifle would, faster to reload than a shotgun, and is smaller and more manuverable than most rifles/shotguns. And as an added bonus, when you are old enough to get a pistol you can get your Sub 2000 setup for most populas handgun magazines. Glock, S&W 58, Sig, Ruger, etc...
"When the pin is pulled, Mr. Grenade is not our friend."
-U.S. Army Training Notice