When visiting a foreign country it’s always a good idea to learn a few words. You probably won’t need to use it much if you stick to the main tourist destinations as you’ll find that many people speak English - especially in Siem Reap, Phnom Penh and Sihanoukville.
However, the people you encounter will, for sure, be impressed if you try to speak a little Khmer with them. Cambodian people truly appreciate when they meet foreigners who try to learn some Khmer words and phrases, so it might well be worth the extra effort.
The best way to learn a new language is to listen and talk to native speakers. This brief little introduction to the Khmer language will give you a head start.