Well, what do you mean you want to "learn Linux"? An OS exists strictly for the convenience of the end-user -- to facilitate his doing whatever he wants to do. What do you use your Windows machine for? Probably your top 3 uses are 1) Internet 2) MP3s 3) Word Processing. I'm sure that you can, right now (with no other knowledge) do all 3 of those things on your Linux machine.
General things that you should learn are:
1) How to install/make/compile/etc. new programs
2) Learn about file permissions and CHMOD (type "man chmod" at a prompt)
3) How to customize and compile a kernel specific to your machine
I suggest
http://www.linuxnewbie.org as a good resource for starters.
Look for the "DOS => UNIX" comparison chart for command line stuff that's similar to DOS commands. A starter list:
ls = dir
cd = cd
rm = del
rm -r = delete recursively (everything in a directory, and the directory itself)
For any command, you can type "man [command]" at the prompt to get a full list of options for that command.