I was looking at Ubuntu. Is that a good one to learn on?
It depends on what you intend to learn, and your computer skills.
To me, "learning Linux" does not mean only learning how to use it at the GUI level. The use of a Linux GUI is similar enough to Windows that you should be able to guess your way through the mechanics of it. Finding things and figuring out what does what is not difficult.
Learning Linux command line will teach you much more. Given that knowledge it doesn't matter if you walk up to a Ubuntu system, or Fedora, or Debian, or Slackware, or whatever. Say for example you wanted to add a new user, you wouldn't need to search around the GUI to find some type of user administration tool. You'd just go to the command line and run "useradd". It's there on all the distros. GUIs just put a pretty point-click face on top of command line stuff that does the real work.
You can learn command line on any Linux distro. Ubuntu would be fine and is very popular. If you are already comfortable with the command line in an OS (even Windows has a rudimentary one), and you want to learn Linux with a leaning towards it's sibling Unix (Solaris, HPUX, etc.) then you might want to consider Slackware. I would not go for Slackware as your first distro if your only experience is in Windows. You might be able to do it, but you would probably struggle at first. Slackware doesn't hold your hand and guide you along like Ubuntu would.
There is no reason NOT to recommend Ubuntu as a first distro IMHO. However, dig in there and use it's command line, don't just limit yourself to the GUI. Treat the GUI as your fallback when you can't learn what you need to do from the command line in a timely fashion.
Head over to
LinuxQuestions.org for excellent help getting to know Linux. The forums are very good over there.