Keepvid.com is a website you can use to download videos from Youtube.
It is really easy to use, just paste the link into the URL bar and press download. You will then see your video with a variety of formats. In order to edit your video on IMovie (Mac) or Windows Movie Player (Windows) it is best to choose the MP4 format.
If you do not already have Java installed you will need to install it before you can download videos. There is a link which you can follow to do this.
Learn more: keepvid.com