Start with Ubuntu, it's easy to install which will give you an immediate environment to get in and "root" around.
As far as learning Gnu/Linux, you need to know how to get around, install things, and read man pages.
Getting around includes things like command line file system navigation (ls, cd -, cd ~, grep, etc.)
Installing things is as easy as "apt-get install" and there's also a nice tool called "aptitude" for searching and downloading things
Reading man pages is as simple as typing "man <thing>" you can even read "man pages" on the command to read man pages
If you're not sure what to read man pages on, just type "ls usr/bin" and start reading man pages on things that look interesting
Since the output of that might be too big to fit in the prompt there's an easy solution that you'll use a lot...pipe it.
"ls /usr/bin | less" will pipe the results of the ls into less which lets you scroll through the text. Read the man pages for more details.
In windows if you ever used the "type" keyword from command line, you'll be using "cat" And you can also pipe that into less or you can just call less on the file.
another thing I tend to do a lot is pipe results into grep. Like, "ls ~/Documents | grep java" prints all the files in my documents folder that contain the
word java
of course multiple pipes are possible "cat war_and_peace.txt | grep peace | less" pipes every line of my text file that contains the word peace into less
Once you're familiar doing those basic things you can continue to accrete your command line wizadry.
Lastly, if you haven't already done so, find all the linux jokes on xkcd...start with this one
http://xkcd.com/456/