How do I get video clips online without downloading anything?

I want to make a short video using clips from other videos, like maybe from Youtube, but I don’t really know how to do it. I use Widows Moviemaker a lot, so I’m good with that, but if I wanted a clip of a girl running through the woods, for example, and I found one on youtube how would I isolate that individual part and use it in moviemaker?

Thanks!

By: Morgan