There are a few apps that you will absolutely need for your trip to Cambodia. These are:
Grab is a ride share app used in Cambodia, as well as Singapore, Malaysia, Indonesia, Thailand, Vietnam, the Philippines, and Myanmar. This is an essential app for hailing taxis, tuktuks, and motorbikes.
Even if you prefer to hail a taxi/tuktuk the old fashioned way, you should still check on Grab to verify the price. Once a driver sees that you are a foreigner, they are likely to double or triple the price from what locals would pay. If you have Grab on your phone, you can tell them the price that Grab is offering or show them directly. They will almost always match the same price.
PassApp is a Cambodian ride sharing app. The prices on PassApp can be up to 20% cheaper than on Grab. You do need a Cambodian SIM card to activate PassApp though, so keep this in mind if you aren't planning on buying a local SIM card.
If you are travelling around SouthEast Asia, 12GO is the go-to app for booking inter-city transport. Simply search where you want to travel to and 12GO will show you all the options. You can search them by departure time, lowest price, and speed to decide when you want to get to your destination.
On a side note, I just realise that when spoken aloud, 12GO is "want to go". I've been using 12GO since 2018 and had never noticed that. Now you know too!
I personally love travelling via scooters. Technically, you should have an International Drivers Permit (IDP) to hire and drive a scooter, but this isn't monitored. It is easy to get around on a scooter, especially if you are experienced and have driven one before.
Bike hire starts at USD$8 for a standard small scooter and USD$12 for something a bit nicer. If you're doing 2+ hour drives (such as to Kaoh Ker temple complex), it may be worthwhile hiring a 125cc bike or something a bit more comfortable than the cheapest option, but that's up to personal taste.
You can barter at scooter hire places. The prices they have listed are not necessarily fixed. Also consider talking with your accommodation provider. If you are staying at a budget hotel, they may hire scooters and you may be able to get a cheaper price for your room and scooter if you barter this before you book.
If it is your first time driving a scooter (which is essentially an automatic motorbike), make sure to watch some videos on how to drive safely. All across Southeast Asia you will notice dozens of tourists covered in bandages due to scooter accidents - hundreds even if you're in hotspots such as Bali. Do yourself a favour and don't be one of them.
If you are nervous about riding a scooter, trust your gut. It is best to learn how to ride when you are with someone experienced and responsible so they can show you how to drive safely.
Key safety things are:
Always wear a helmet
Never drive while intoxicated
Go slow and keep in control
Don't overtake locals. If they're driving slow, they may know something that you don't
Ask if the hire company rents protective kneepads and arm guards
Wear glasses (!!!) so you don't have stones and bugs fly into your eyes
Avoid driving at night - and if you have to, bring clear glasses to use as protective eye gear (hire helmet visers are often too scratched to see through)
Ask the hire company to show you how to drive safely and familiarise yourself with driving in a quiet area before you reach busy roads
Avoid highways and places where people are 80+ kms per hour - you do not want to be struck by someone else
You can hail Tuktuks easily with Grab or with PassApp.
Even if you prefer to hail a tuktuk the old fashioned way, you should still check on Grab to verify the price. Once a driver sees that you are a foreigner, they are likely to double or triple the price from what locals would pay. If you have Grab on your phone, you can tell them the price that Grab is offering or show them directly. They will almost always match the same price.
You can hail taxis easily with Grab or PassApp.
Even if you prefer to hail a taxi the old fashioned way, you should still check on Grab to verify the price. Once a driver sees that you are a foreigner, they are likely to double or triple the price from what locals would pay. If you have Grab on your phone, you can tell them the price that Grab is offering or show them directly. They will almost always match the same price.
Use 12GO to find busses between cities! You can have a look at the pictures of the bus as well to determine if this will be comfortable or not.
If you haven't yet tried a Sleeper Bus, I strongly recommend it between bigger cities. Do note that the more expensive sleeper busses may have running water, charging ports, thicker mattresses, blankets and pillows etc. The cheaper sleeper busses may have none of this and may be very creaky. Consider your level of comfort when booking a sleeper bus.
If you want to get to the islands, you'll need to go to Sihanoukville to catch a ferry. There are busses from Siem Reap and Phnom Penh to Sihanoukville, as well as from other smaller towns (look on 12GO to find out how to get there from your starting destination). Sihanoukville also has an airport.
The main port that takes passengers to the islands is Sihanoukville Autonomous Port - circled in green in this screenshot of Google Maps. There is also a red port that shows several ferry lines departing to the islands - this port actually closed a few years ago, so don't go there!
You can find ferries on 12GO and make bookings through the app.
Personally, I like to find a ferry time that I want, and just rockup and pay for the ferry in cash when I arrive. I've never missed a ferry and I've never had an instance of a ferry being booked out (without there being another one immediately after with another company). It's often cheaper in cash, and this eliminates any stress of arriving on time. Personal preference though.