Since you're still in high school I would recommend holding off on buying a firearm until you're at least 18. You are able to get a shotgun at your age.
When I was in high school there was a super sketchy guy that could sell you all types of things. He even had a cheap hand gun for $75, as long as you were okay with it having a body count already.
I also knew a guy who went to a gun show where the rules are not followed all the time. Many people will sell an underage person handguns and other firearms they're too young for.