I'd like to second what Ron said. Pump actions are the way to go. Much more reliable than most semiautos. And you won't go wrong w/ a Remington 870. The 870 comes in a model chambered for 3 1/2 inch loads, which you might want to have for turkeys, or geese. But in such models you certainly don't have to use the big shoulder-crushing shells, the little 2 3/4 inch dove loads from Walmart work just fine too.
I'd also say that you should get a 12 gauge, unless you're a pretty small-framed guy and don't plan on growing any more, but at age 15, I'd think you will. 12 gauge just throws more lead out there, enough to be worth it in my opinion.
Lots of wing shooters like break actions, but I'd really recommend you consider a pump. They're pretty simple, versitile, reliable, and you'll be surprised at how fast you'll get used to it.
Good luck with whatever you decide!