Anyone going to SR will no doubt do a lot of research but below are some short hints.

When to go: Dec- Feb are probably best. Dry and getting really hot even by mid Feb. Would not go in wet season starting in June. Endemic for dengue fever then.

Money: The $USD is official currency unless < $1 when you will get Cambodia Riep. This is a poor country so whenever possible get $USD in small denominations, $1-5. ATM will likely give you large denomination. One fix is to use only an ATM at a bank during business hours. After you get your money go inside and ask for small bills. No charge. All ATM charge about $4 even if you think you have a no charge card.

Water: Best to use bottled even for teeth.

Language: Khmer (pronounce koomai). Not a place where everyone speaks English.

Town lay out. Siem Reap River runs N/S thru town. Angkor Wat is a few miles north of SR. Tonle Sap is about 10 miles south. Airport is to the west.

Airport to hotel. Cab ( about $10) or Tuk Tuk (little motorized cycle taxi) about $6. Just depends how much luggage you have.

Tuk Tuks. Use them! Do not be afraid. Not a rip off as in other parts of Asia. You will be asked every 10 feet if you want one. $3 to go in and around the central part of city. You may be asked to bargain a bit. For a longish ride you may pay $4. They are fun, safe. Carry a little city map with you in case you are going to a place the driver may not know.

Hotels. Vary from very high end to budget. You just need to figure out what you want. Somewhere in central area may be best so you can walk around. 

Pub Street, Old Market, Night Market. Very touristy but you have to go. Bargain in the markets but do not get irritated. When you think about what you are paying vs hourly wage to make it you are getting a deal.

Wats (temples). Many all over the city. You can walk in. Removes shoes and hat if you go inside a building. Monks are approachable and fun to talk to. Get a blessing and pay a dollar. Might help!

Food: Obviously mostly Cambodian . Lots of indian too. And some western type. There is a favorite at north end near Wat Po Lanka. Many really good Cambodian small dishes you will not get elsewhere. Plus they are training disadvantaged students for restaurant work. Get a reservation if you go after 7. Il Forno in Pub St area. Great pizza!  Sugar Palm. Nice Cambodian on 3d floor balcony so nice breeze.Curry Walla. indian near Prince D'Angkor hotel. Good lunch spot. New Leaf Bookstore near Pub St. Good food plus a movie every Monday night about something related to Cambodia.

Angor Wat. The reason you are here. Arrange a tour to go for sunrise. You will go in, see sunrsie over temple, then back out for breakfast and then in again for tour. You can go with a Tuk Tuk driver but find a guide from a good tour company who really knows history. More expensive but worth it. Call 2 days in advance if possible. Visit to National Museum is also a good idea prior to going.

Tonle Sap. Lake south of town with floating plus famous stilt villages. A must see. The floating village nearist to SR is a tourist trap. Go to stilt villages a bit further SE down the lake. The stilt villages are on streams about 1 km before lake. From these villages the tour will take you in a boat to see floating villages where stream enters lake.

Hot and tired? Try famous Singapore Sling at Raffles.