I would suggest slackware or gentoo if you want to learn stuff, or Ubuntu if you just want to use things.
The Linux Documentation Project
is a good place to start.
Also, once you get linux installed there's two wonderful reference programs, 'info', and 'man' (short for manual).
At a terminal (probably an Xterm) type "info info" and hit return. That will tell you all about how to use the info program. "info coreutils" is a good place to start learning about the basic unix/linux commands. Also if you come across a specific program that you would like to know more about use the 'man' command. For example to learn about the man program type 'man man' and it will tell you all about 'man'. If you really want to learn about some commands and programs just 'cd' to /bin and then 'ls' the contents of that directory. /bin contains most of your basic programs and you can just look up information about them using 'man'.
Sorry if that was a bit confusing but most of this stuff should become second nature to you in time.