Get your VISA (everyone needs one for cambodia) on the Airport upon arrival, saves you time getting to the Cambodian Embassy here at the Gateway (takes at least two visits). At the airport you can get the VISA for the same price (think it was US$20) and bring two passport pics. If you don't have pics you pay an extra $1 for the VISA.
For lodging and tours and getting drivers, go to the Tpurist information desk on the left, after you passed through the Immigration. There is a brown wooden (wood is brown...) counter on the left, and the lady there is very helpful in arranging for you almost everything you need (she speak english as many people there do or are eager to learn):
Hotel, boat trip to Siem Reap (recommended, and sit on the roof of the boat), hotels in Siem Reap, they have pics of the hotel rooms for you to choose, drivers for you to show you Phnom Phen or Siem Reap or other places, etc..
In Phnom Phen it is good to walk around, at night some streets can be very dark, they don't switch on all lights. Or get those motorbike drivers to bring you around, let them earn some money please (negotiate, but common price is about US$10 per day).
For eating on the road side you have to be adventurious (many insect snacks, lizards), or get your style of food in the many restaurants. Visit the Toeul Luong 'school' one of the many torture paces where the POL POT interrogated and killed people and the many killing fields around the city.
In Siem Reap: get a driver to bring you to the temples. Buy a book on Angkor wat, it has a yellow cover with a pic of Angkor wat (about 12 cm high and 10 cm wide). Forgot the writers name, but this is the best book on Angkor wat you can get (buy it from those schoolchildren around the temples, they sell it for a few US$ per book). You need at least three days to see most of the temples, so worthwhile to get a three days pass (US$40) AND do NOT cut the pass smaller (to get it fit into your wallet)!! We did it and they got angry at us for doing so, they need the extra piece of plastic for punching the holes everytime you pass the gate. But for the most part people there are VERY nice! especially the children! They work there selling books, toys, ikat weaving etc. to earn money for going to school! SO buy lots from them!!!!!!
AND: bring LOTS of US$1 notes, as this is the minimum amount you need to buy all kinds of stuff: drinks, snacks etc. Everything is sold in US$, so no need to bring Cambodian money.
Have a good time and buy lots from them to support the country and people!