Know Before You Go
The temples of Angkor are a sacred religious site, and visitors should dress appropriately. Shorts should be at least knee length, shoulders should be covered. Respect the 'No Entry' signs. They are there for your safety as well as for the protection of this UNESCO world heritage site. Don't climb all over the ruins. Ask politely if you wish to take photos of people, especially the monks. Do not litter. It's all about respect really.
You need a ticket to gain entry to the park. As prices do change, it's best that you check the official website here for the most up to date information.