Audio. In my opinion, this is the most important component of your early films other than story and characters. Next time you watch a film, close your eyes. You’ll be able to tell how much of a budget the film has just by listening to the audio. You can get away with bad video. You can’t get away with bad audio. Just look at all these found footage films. Horrible video quality. Decent audio.
One of the worst things you can do is believe in the quality of your built-in microphone. Big mistake. All built-in microphones are horrible. You can pick up an external microphone, for under $100, which can plug directly into your camera. It will automatically improve your audio situation. I recommend also getting an external audio recorder as well. Something like a Tascam or a Zoom recorder works great. You can get small ones for around $99 and work your way up to spending several hundred dollars. This is your best bet for sound. This way you can get nice and close to your subject without worrying about being attached to the camera. A boom pole can also help a ton.
I’d say for your first film to just stick with the external microphone plugged directly into the camera. This will work fine to start with, and it won’t break the bank either. Be careful, however, as some camcorders don’t have the jack for a mic. However, if you are willing to spend an extra hundred dollars or so then you should definitely pick up an external audio recorder as well. Your film will become much more professional. You have to have the story and characters to make people want to watch your film. That is a given. If you also have the audio to deliver the goods to your audience? Golden.