There is no doubt about it, Khmer is a difficult language, but PST is absolutely the best way to go about learning the language. The links below will walk you through the Peace Corps language process from before staging to after arriving at site. You can find information about how language training works in PST, basic introductions to the Khmer language and the benefits of getting a language tutor at site.
Introduction to Khmer Language
Learning a Language in Peace Corps: What your language training will look like
Cambodia: More Than Just Angkor 