Angkor Wat is one of the most amazing places I've been to. It's very visitor-friendly and you don't really need to do much to prepare for it (aside from reading up on its amazing history).
Here's just a short list of things you should bring with you:
- Ticket: duh.
- Water: Siem Reap is very hot and Angkor Wat is very big requiring lots of walking around in the sun. Bring lots of water and stay hydrated. I recommend at least 1 liter of water (1/3 of a gallon)
- Sunscreen: wear at least SPF50 sunscreen, and if you're extra sensitive wear SPF70-90. Replenish every few hours between 9am and 5pm because you will sweat them off throughout the day
- Insect repellant: lots of mosquitoes around Angkor Wat and especially active in early evening. Also lots of other bugs I don't even recognize. Very handy to have some insect spray
- GPS: Angkor Wat is massive. I found my phone's GPS really helped me navigate around
- Camera: duh.
- Backpack: with all the stuff you need to bring, it's better to just put everything in the backpack
- Passport: they didn't check mine, but I've heard from some people that they check. So bring it just in case
This is all I brought with me. I'm sure someone can suggest more stuff I missed