If you're looking to download videos from YouTube, it's actually really straightforward these days! Here's how:
The first thing you need to do is download a software program. There are a few different ones around, but they all pretty much do the same thing, so it doesn't matter which one you choose.
Personally, I use YTD You Tube Downloader. It's super easy to use, all you need to do is copy the URL of the YouTube video that you want to download into the address bar in YTD and press Go!
Remember- video files are large so I would only do it for shorter videos, or ones that you definitely want to keep, or watch again, otherwise your hard drive will fill up pretty quickly.
You should also be aware of copyright law which forbids you from downloading and sharing copyrighted material (movies, TV shows, music videos etc.). Although downloading videos through YouTube should escape detection, just be aware if you plan to share these videos through filesharing sites.