All travellers (except those from Laos, Malaysia, Philippines, Singapore, Vietnam, Thailand, Indonesia, Brunei Darussalam, Myanmar) will require a visa to enter Cambodia. However, the process does not seem to be onerous. A visa can be purchased at the airport on arrival, or an electronic visa can be purchased on-line before travel (applyat least 3 days before departure). The e-visa can be acqired here: https://www.evisa.gov.kh/application_new
Visitors from the following countries will need to apply for a visa from the Royal Embassy of Kingdom of Cambodia in their country before leaving: Afghanistan, Algeria, Arab Saudi, Bangladesh, Iran, Iraq, Pakistan, Sri Lanka, Sudan, Nigeria.
The cost of the visa is:
Tourist visa $30 USD (valid for 30 days)
Business visa $35 USD (valid for 30 days)
you will need a passport which has at least 6 months validity, and a recent passport-sized photo.
A departure tax is charged on all domestic and international flights. the cost is $25 USD
For more information see: https://www.tourismcambodia.com/tripplanner/essential-information/visa-passport.htm