Learning a new language is never an easy task, but it may often seem all the more difficult if your first language is an Indo-European language, like English, and you wish to learn a non-Indo-European language, like Khmer. However, it is a much less difficult task than it might seem. With patience, repetitive practice, the right information and a will to fulfill your task, you can learn Khmer by yourself.
Make sure your computer has the Unicode characters for Khmer script enabled. Depending on your operating system and your web browser, this may require one of a variety of actions. (See Resources 1)
Acquaint yourself with the International Phonetic Alphabet (IPA). (See Resources 2) IPA is an indispensably useful tool when learning new languages, as it provides a universal phonetic alphabet meant to display various sounds. Learning which sound is associated with an IPA symbol will be very important.
Learn the Khmer alphabet. The knowledge of IPA gained in Step 2 will allow you to fairly easily learn what sounds each character of the Khmer alphabet represent, with the aid of such resources as Omniglot. (See Resources 3)
Study Khmer grammar. Khmer is a very different language from English, with very different grammatical rules. Several Internet resources such as Mylanguages.org (See Resources 4) provide free lessons in Khmer grammar. Pay special attention to these lessons, as knowing the grammar is as important as knowing the vocabulary.
Start picking up vocabulary. Don't just memorize the words as a list, but start using them to construct sentences as soon as it becomes possible. Immersion tends to be the most successful method of building up vocabulary in a new language, so start using the Khmer vocabulary as often as possible.