A video on a computer will be in a certain file format.
The common ones are AVI, MPEG, MOV (Quicktime) and so on.
Youtube will probably expect a video clip in a certain file format (avi or mpeg for example). Check the web site and it will probably tell you.
You need to "rip" the video off the DVD and get it in the right format for youtube.
There are tools to do this, but as I have never done it I cannot suggest one.
If you have Nero or similar product, that may do it, or maybe Windows Media Player will do it.