hello. i hope i am posting in the right place.
i have tried to do something with this:

i wanted the pylons to blast the boat at the end of the video and show a wonderful, invisible shockwave.
but i found myself creating, albeit in 3d, a full scale model of the lake.
i cannot get the perspective right. or is it the camera position and lens?
i use the video as a background image, as a guide. do i need to play the video on a plane, and include it in the scene?
i can’t get my head around it.

any help would be greatly appreciated.

you would only need to composite the things you are adding to the video
ie - you wouldn’t need to model the lake as it’s already in your video

I would start with a lightning bolt tutorial
(somthing like http://www.selleri.org/Blender/tuts/Lightnings.pdf - there are many more)
and then
search for compositing tutorials

if you are new to blender I would start with a few very simple tutorials before tackling this one