First, I encourage you to use a search engine and browse the Ubuntu knowledge base. Searching for "play dvd ubuntu" brings up the standard help document: https://help.ubuntu.com/community/Re...ts/PlayingDVDs.
Start by installing ubuntu-restricted-extras by opening a terminal and typing the command:
Then read the section in the documentation above on installing libdvdcss.
sudo apt-get install ubuntu-restricted-extras
BTW, my preferred video player is SMPlayer. You can install it with the command:
sudo apt-get install smplayer