If you are physically in Cambodia, immersion is your best friend.
Because Khmer isn't on mainstream apps like Duolingo, you’ll need to look at more specialized resources: 1. Language Schools & Study Groups Learn Khmer (Cambodian)
Unlike Thai, Vietnamese, or Mandarin, Khmer is NOT a tonal language . You don't have to worry about the pitch of your voice changing the entire meaning of a word. If you are physically in Cambodia, immersion is