You need to use a video downloader, either an extension to your current browser (depending on what that is, eg, Safari, Firefox, Chrome) and depending on the source, eg, YouTube or other some extensions are for specific sources.
Alternatively you could try a third party app like ClipGrab.
What you seem to want to do is not "super" difficult but does require a bit of acquired knowledge.
You are really trying to do two things;
1. Download video content from the internet. This has its own pitfalls and some content may be restricted or copyrighted.
2. Import/export said video to your YouTube channel. I'm sure you would find lots of info on this process on YouTube itself.