Once you arrive in Siem Reap you're going to feel super pressured to jump into the temples immediately, from everyone and everything. But stop, take a minute, and start your trip at the Angkor National Museum.
We visited the museum on our last day in Angkor, but I kinda wish we did it on the first day. Do the audio guided tour, visit each station throughout the museum which is a 4-5 hour tour, and then get your 3 day temple tour with an official guide. We did it the other way around and it was extremely educational, but as native english speakers it would have been a better primer (in an air conditioned museum, no less) to get our feet wet in Angkor history. More