Just checked out your portfolio. You're outstandingly talented for someone so early in their artistic journey; if I was at your level when I was sixteen, I'd probably be an insufferable egotist, so respect to you.
As for commission work, a PayPal account is fairly straightforward to get going; I don't know what sort of costs you have to pay to use it in a professional capacity, though. It's a great system, but I imagine that it comes at a price. As for what to sell, I think pretty much anything you can draw well is fair game; the real money comes when you step out of your comfort zone.
Happy New Year!