I'm no video professional, but I've had the need to make videos with nice audio. If you noticed, the audio from the internal microphone of the camera is really crappy, you can hear everything except your own voice.
After looking for some time I founded a really good solution. If your camera has a mic input, just get a lavalier microphone and connect it to your camera, that will improve the sound a great deal.
But what if you don't have a mic input on your camera, well, that situation just require another piece, a digital recorder, connect your mic to it and now you can record audio externally from your camera and also have the freedom to move to where ever part of the frame without the sound changing.
If you don't have the money for a digital recorder, just use your mobile phone to record all the sound, it will be almost the same.
Then a bit of fine tuning in a program like Audacity and your audio will be ready to use.
Now all you have to do is to sync your audio track with your video on your video editor, that's not that hard. When you start recording your audio and video, just clap one time before starting, that way that peak of sound will register in both audios, the one from your camera and the one from your digital recorder. Then, in post, just align that peak and delete the sound from the camera and voila.