Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
The best way to learn Linux is to use it. A good introductory book will help, but you won't really learn Linux or become confident in your skills unless you park yourself in front of a Linux system and start typing (and popping windows open, creating files, find your way around the desktop ...). And one of the easiest ways to start using Linux -- even before you've committed hardware to it -- is to run Linux in "live" mode. And one of the best ways to run in live mode is to build yourself a bootable USB drive using an excellent tool called UNetbootin.
To run Linux in live mode, you install a bootable release on a USB drive or DVD. Your hard drive is not impacted unless you decide that you like the Linux release that you're trying enough that you want to commit it to disk. This means that you can run Linux as much as you like without any risk of overwriting or affecting your currently installed operating system. The only down side to running Linux in live mode is that it won't save files that you create or changes that you make while you're logged in. Every time you boot from your USB drive, you return to the same original system.
UNetbootin (Universal Network Installer) is a superb little tool that makes it dead easy to create bootable USB drives from any in a large selection of available Linux distributions.