Sub amps are typically wired with 4awg or bigger, because class D mono amps usually pull a lot more power than say a 2 or 4 channel amp for normal drivers.
You really shouldn't need to worry about upgrading your alternator unless you're gonna be going crazy with your setup, but then again if you were going to be doing that you wouldn't be here asking a question like yours. I always recommend upgrading your battery/ground cables under the hood though, which I myself have yet to do (btw those are typically 0awg).
That being said, all you really need is a good 10- or 12-inch sub for decent sound. No need to overdo it and rattle the car to pieces and annoying people by blasting a couple huge Jackhammers.
One last note: read up on the subject A LOT if you intend to do the install yourself, and if at all possible get a friend to show you how to do it that's done it a few times before. The install is always at least if not more important than the equipment itself.
