this should probably be in video software help, or general video.
anyway, dowloading footage to make amvs is highly discouraged.
and if you think that takes a lot of space, you haven't seen anything yet.
to get started, I suggest that you
Read ErMaC & AbsoluteDestiny's Friendly AMV Guides
it's long, and a bit techy...but it's a very good guide.
the other thing I reccomend that you do is to look at other people's vids to see what they do (to kinda give you an idea of how to edit your own vids)