given the law, I'd say don't get an AK-47. You are only 15 years old, and therefore not even old enough to keep the gun in your room (that would count as possession, going by federal law). What would be the point of getting a gun that you can't even touch without an adult present?
Another thing to think about is that the AK is not very ideal for home defense. The bullets (7.62x39mm in particular, since the models you are referring to are chambered in it) will make a hole through the bad guy and most of your/your parents' house. That is a major liability issue should the bullet go through the outer wall of the house and end up in a neighbor's car, house, or worse. Although softpoint or hollowpoint rounds may reduce the risk, they won't stop the bullet from going through a wall or two or three if you miss - or even if you hit your intended target.
You'd be much better off with a shotgun, as said. You are legally old enough to possess one (though you cannot buy it yourself), and be the actual owner of it via having it gifted or otherwise transferred by your parents or family member. Not only does that make things concerning the law much easier, but a 12 ga. loaded with 00 shot is going to be much more effective at HD range without the huge liability risk from overpenetration.
for a shotgun, I'd suggest a Mossberg 500. Very affordable, rugged, and reliable.
Another thing to think about is maybe just getting a basic Saiga. It is not legally an assault weapon unless you convert it. You'd be able to own that, and could simply convert it in a couple years when you're of age to possess a 'real' AK. The best part? You can get one in other calibers (they are available also in 5.45 and 5.56, which are much more suitable calibers for HD)
