Some people say that the audio is actually more important than the video, bad audio will immediately put people off your video.
But its super easy to get good audio, listen to the audio in my latest video (channel trailer) it sounds great, but the room I recorded that video in is far from quiet, it has constant traffic outside and several computers and other tech running, its actually quite loud up there and most camera microphones seem to pick it up really easily.
Which is why I do not use the built in microphone, you need to use an external microphone for example a usb microphone, a microphone you plug into your camera or even a lapel microphone plugged into a phone in your pocket. Unfortunately for me the Panasonic DMC-G5 does not feature a microphone jack and the onboard mic is pants, so I used the only other microphone I have, my amazingly good Audio Technica AT2020 USB microphone.
This is a pretty cheap microphone coming in at just over £100 but you do not have to use this exact microphone to achive great results, the is many other microphones on the market. One example would be the Rode SmartLav+ it is a lapel mic so it mounts on your t-shirt, and plugs into a smartphone or iPod to record the audio, at £50 you can not complain as the audio quality is top notch.
If that is still too expensive you can try the Speedlink SPES lapel, not only does it sound good, its under £10 you would need a separate recorder though as its not compatible with smartphones or any device with the combined headhpone and microphone jack.
But you can get a cheap Sony pocket recorder for £30 to record the audio from that mic.
So now that you have good audio recorded with your video all you have to do is sync it up in editing, this is very simple to do in Final Cut Pro X and should be just as simple in Adobe Premier.
In FCP just import both the video and audio clips, select both in the media area, right click, and select new synchronized clip, the default settings worked very well for me so just click ok and it will then make a new clip you can drag onto the timeline which will have your great audio synced up perfectly with the video. One last tip, when recording audio in videos you want to get the microphone as close as possible to help reduce background noise, in my case I had my AT2020 stacked up on some speakers just out of shot to the right of me, if it was any higher it would be in shot.
This audio was surprisingly very clean and I did not even do any noise reduction in adobe audition, so that is a really important lesson, get the microphone as close as possible, this is one reason why a lapel mic seems like the better choice but I had the AT2020 already so I used it and it worked quite well. I hope this information helps someone who is struggling with audio, it does seem very hard when you first start but once you learn some of the basics its actually quite easy.