For starters Ubuntu has setup an online guide at
www.ubuntuguide.org. It is good enough to learn about the basics of how to operate ubuntu.
And eventually you will need to work a little bit with command line if you want to exploit the full potential of linux. For that you can either read online documentation at
http://www.tldp.org/guides.html or get hold of some books on linux system administration, since the basic commands of linux ae the same.