Just about any distro will do to get your feet wet, but eventually you'll probably want to sample several distributions before you settle on the one that suits you best. That's the beauty of it; they're all "Linux" under the hood, but the great variety in the exact implementations lets you find the ones that are best for you, because there's no one version that's right for everybody.
Go to DistroWatch and take a look at the "Major Distributions
" page, and simply choose one that sounds good to you. The best thing is to have a spare computer that you can play with and install and wipe as needed. But dual booting or running a live disk version are also good ways to get started.
My main suggestion for you, if you really
want to learn Linux, is to stay away from the "beginner-friendly" distributions, like Mandriva or Ubuntu. The more hand-holding a system does for you, the less you really learn. Choose something a bit more complex, and be prepared to take the time to sweat through the hard parts. In the end you'll be glad you did.