The RODE Videomic or VideoMic Pro is a good option to improve the audio of your videos easily. I have to disagree with the comment about the Pro versio not being worth more as this is not correct.
The VideoMic Pro has an integrated +20dB setting which boosts the signal of the mic and produce noticeable better results.
But a camera mounted microphone is not the best option really, getting the microphone as close as possible to the talent is always the best option for good quality audio.
You can check my channel, we have various reviews of audio equipment from RODE (including the VideoMic Pro mentioned before), JuicedLink, Beachtek, Zoom, etc. We show many examples of their uses, they should all give you a good idea of what each type of microphone does, what a preamp does, etc.
Hope this helped.
Cheers!