The South East Asia is a great region to travel in.
If you plan to go to Cambodia you need visa.
You can get visa on arrival, but I recommend apply for e-Visa online before you go.
It have several advanges to have ready before you arrive.
No queing, no hassle at border especially if enter and crossing a land border.
Check out : http://evisa.mfaic.gov.kh/
Apply , pay by card and print it a couple of days later when it is ready.
I travel with e-VIsa myself.