Install it and use it. Start with Ubuntu if you've never used Linux before, but you'll want to move to a different distro if you want to become a "guru" as Ubuntu is designed for the Linux newbie, although it is fine for any Linux user, you'll learn more on a distro that isn't specifically designed for ease of use.
Read about it. If you've never used Linux before, research what you need to know before installing it(how to install, how partitions work, how to install software, basic command line use, understanding configuration files, etc.). Read news sites, blogs, wikis, forums, etc. and utilize these resources. Don't be afraid to ask questions. There's tons of info on Linux out there and Google is your best friend in this case, whether you are new and looking for info or experienced and troubleshooting a problem.
In short, use it, read about it, play with it.