To me, Linux means command line.
Not all the time, but since I switched to Linux a few weeks ago, I've been doing a lot more system stuff than I ever did with Windows.
I've been using Linux for several years, just not for my computer. There were too many things I did in Windows that could be done very easily in Linux, but that changed a lot in the last few years.
A good site to start with for learning shell is
http://linuxcommand.org. Most important rule for learning command line, in my opinion, is to never be in shell as root until you have a good idea what you're doing. Root can do a lot of damage real fast. Let's say you want to delete all files and directories in a temp directory in your home directory, but want to keep the temp directory, and you're logged in as root.
Command would be:
rm -Rf /home/username/temp/*
Now let's say you weren't thinking clearly, maybe you thought specify directory, then add what specifically, and you typed:
rm -Rf /home/username/temp *
Huge difference there.
If you're not sure what that will do, then don't log in as root yet.