if you're vidding in iMovie, i can give you some more specific links. these are just general ones. some of these are quite old now, too.

vidding for newbies
vidding newbie cheatsheet
vidding 101: how to get started
vidding: the creative process (discussion in the comments)
vidding workshop

having made my first (and only, so far) vid last year for festivids, i can say that for me the best process was to learn how to use the vidding program, learn how to make clips, and then just mess around and see what happened. it's not an intuitive process for me at all, which is why i haven't managed to finish any of the three other vids i've started. but i keep hoping it'll get easier. :)