Sorry, but QuickTime Player cannot play video DVDs; QuickTime does not include the decoder necessary to decrypt the copy protection. You'll need to play your movies using a DVD player; most PC vendors license and include one with their systems.
Converting such movies for use in iTunes would require breaking that copy protection which is illegal in the US. This being a US-based forum, discussing such practices is not allowed here. If you're elsewhere in the world where the unnecessarily-restrictive US copyright laws don't apply, search the web and you'll find tools to make the conversion.
Sorry we can't help further.