You would have to ask James Cameron, Avatar's writer, director, and producer for an answer, and I'm unable to give you a link to any comments he's made that directly answer this question. However, my opinion is that Cameron, as a well-established environmentalist, would say something like: honor and respect yourself, your planet (although Pandora is actually a moon), and all living beings. I think the lesson here is that if you follow this basic concept your life, and the lives of others, will be rich, balanced, and self-sustaining.
Update: After some searching about, I found this link:
where Cameron discusses his message and the moral lesson of the film.