Cambodia is very hot (humid) and the culture shock can be overwhelming.  You may see poverty first hand and for the first time - and it may be shockling to you.

Because of the heat - take your time and pace yourself when you go out for day trips - you can take a few days instead odf trying to do everything too fast - the heat will slow you down, Remember to drink lots of water (not tap water).  You could take water sterilisation tablets or buy a Steripen (cleans water by ultraviolet light) so that you are not contributing to the land-fill of empty water bottles.

Westerners are used to having problems dealt with by social services that are accountable - this doesn't happen in poor countries - problems can occur and there may be no services to help you, so becoming vigilant and making good decisions becomes important. 

Recommendations for places to stay and for reliable and trustworthy drivers are paramount.

Learning the hard way in Siem Reap could mean being stuck with a 'free  driver' who may not leave you alone (literally) after you have used their 'free' service to a guest house (free = you are obligated).  While many people are poor and will do anything to earn a living,   this is a typical situation that happens a lot.

Finding a reliable & friendly driver and a great (safe) place to stay becomes important.  A good driver will oblectively recommend a place to stay.

As with any new country, there are places you know not to go to (instinct) and  people that you would prefer to not be around. Trust your instincts and spread the word when you find locals who are willing to help you.   Have had friends who have gone to Cambodia, emailed Ra in advance and had an amazing time with him also.  

Some tips for you:

Leave expectations behind; Enjoy (find the positive in everything) your travels, learn by your mistakes,  be kind to everyone, leave quickly when it doesn't feel right, leave the place a little better for you having been there (you will find a way).