After landing at Phnom Penh International airport, exiting the airport is pretty easy. If you already have your visa (through Cambodia e-visa service) go straight to the border control counter. If you do not have a visa, you’ll have to request one at the airport. It is very easy, you have to drop your passport with a picture (if you do not have one they charge you $1) and then wait few minutes to get your visa, it is not free, neither the one online. At the airport is about $20.
Once you pass the border control counter, you’ll see several shops that sell tours, hotel, etc. One thing I recommend is to buy a phone SIM card. I bought a Cellcard week plan for about $6, it included 1.5GB and was pretty useful. I could communicate with my tuk tuk driver, or even just calling a local number to reserved, book, you got it for anything.
One you are all set and exit the airport, the next thing is to get to your hotel, guesthouse, etc. If you do not have a hotel like I did, ask the taxi or tuk tuk driver and tell them your price range. They will find something for you. Once you get to your accommodation, don’t hesitate to refuse to stay and ask the driver to bring you somewhere else.