Sorry, people, but there ain’t no easy answer to that!
If you want your movies to LOOK better, I would say that lighting and camera composition are the most important things.
If you want your movies to be more ENTERTAINING, I would say writing and editing are the most important things.
None of these things have simple catch-all solutions, so I recommend that you check out the Knowledge Base and the forums for some hints and tips.
And make your movie…it doesn’t get made on its own.