The official currency of Cambodia is the Riel. There are roughly 4,000 riel in $1 USD. USD is widely used in most places in the city. You can pay for anything in US dollars, but you will likely get change in riel.
Vendors are often picky about bills they will take when it comes to USD. They want crisp, clean, new bills. Feel free to be as exacting when someone gives you change in USD.
The best way to get local currency, and USD is to make withdrawals of US currency at ATMs (there are many at the airport and around town), and then get change in riel when you begin to make purchases.
Try to match the denomination of your bill to your purchase; for example don't pay for an 8000 riel purchase with a 100$ bill. In general, the USD denominations in circulation are 10s, 20s, and 100s. If you try to use 1$ or 5$ bills, you MIGHT have difficulty.
Here are some basic US dollar / Cambodian riel conversions based on the denomination of the riel bill.
(Keeping in mind 4000 riel = $1.00 USD)
1000 riel = $0.25 USD
5000 riel = $1.25 USD
10,000 riel = $2.50 USD
20,000 riel = $5.00 USD
50,000 riel = $12.50 USD
Credit cards can be used at most sit down / air conditioned restaurants, hotels, and shops. In addition, a credit card can be linked to the Grab app for ease of travel.
Here is a fairly good article about money while travelling in Cambodia.
Tipping: Tipping is usually not expected, but is very much appreciated, as restaurant wages in Cambodia range from $180 - $250 per month.