There are plenty of training courses, but why not just have a go and improvise? Start simple with Movie Maker on your PC, and just work out how to trim and reassemble clips, do simple transitions, maybe add some captions.
Keep your projects short - a minute or so is plenty. Don't bother with fancy titles, just get straight to the point. When filming, audio quality is just as important as the pictures. Perhaps more.
For voiceovers and watermarks you'll want something more powerful than the bundled freebies.
All the NLE packages offer free trials, worth checking out. I personally found Avid rather hard to get to grips with, quite liked Premiere Elements (cut-down version of Premiere Pro but absolutely powerful enough for anybody that isn't in an actual studio) and currently use Vegas Pro which is similarly plenty good enough.
They all have a steep learning curve and can be baffling without training, but there are lots of how-tos on YouTube etc.
If you look around it's quite possible to get slightly older versions as official free downloads. Haven't looked lately but there may also be fully-functioning versions as magazine covermounts.
If you're stuck, I offer competitive rates for simple video editing projects...
R.