Officially, you are not allowed to bring any food or water bottles into the park, and must check these in at the luggage storage at the entrance. In practice, however, bags are rarely searched, and most people have no problem getting a small bottle of water and some snacks in with them, which you'll definitely want, especially if you're planning to stray from the central set of ruins. Buy these beforehand, as they're much more expensive at the site itself. Don't even think of leaving a shred of trash behind you.