RE: 13 using a 308
It's a great rifle, I have a bolt-action Remington .308, and I see no need for any other rifle in the near future for me, since all I hunt with a rifle are Whitetail, Boar, and might even try black bear this December, all of those, the .308 will do fine, my rifle is really accurate, and I have gotten pretty accurate with it, since I was going to the range almost every other week or even sooner.
Like others said, practice, practice, practice!
If ammo being expensive is an issue, then I have a great ammo suggestion for you, try shooting the Federal Powershoks, you can find them at Wal-Mart, make sure your Mom or Dad buy them for you, because a 13 year old isn't eligible to buy ammo, but you canjust give your dad the price of the ammo in cash before you walk up to the counter, and he'll pay with his credit card, or with the cash you gave him.
The Federal Powershoks are cheap at Wal-Mart in comparison to other ammo, you'll be able to tell the Powershoks from the rest of the ammo, because of the dark, blue box.
Good Luck!