Thanks, Chelle! To use movie clips, first of all you need to get them into your computer. I do this in two ways: "rip" a movie from a DVD using a program called Handbrake. The program is a free download from the web and you can use it to copy the entire movie if you wish or just a chapter into your computer. The smaller the file you copy, the less time it'll take to render the clips you want to use in your ukulele video. The other way to get movie clips is to download short clips you can find on YouTube. Use the website savevid.com to download YouTube videos.
If you have a Mac like I do, you can use iMovie to edit these clips. I actually use Final Cut Express to edit my videos, but I still use iMovie to convert movie files into a format that Final Cut can use.
I've learned quite a bit about video editing by trial and error, but there are also quite a few video editing tutorials on YouTube.
By the way, if you use movie clips, you might want to stick with older movies. You're less likely to run into copyright problems. One of the blackmarks I received on my old Keonepax channel was a cover of the theme song from "Princess Mononoke." I used a couple of minutes of clips from that movie for the visuals in my video and got nailed for copyright infringement.