I did this twice before I was 18: I just called and booked the room with my dad's CC over the phone, and when I checked in I just asked if I could pay cash (I looked like a fine, upstanding citizen in my X-files t-shirt and jeans, heh) for the room. Don't touch the mini-bar or TV, though, or they'll charge it. They might just ask you to pay for it when you check out, but better safe than sorry.