Did you purchase or download a distribution yet? Do you have free disk space you can allocate to your GNU/Linux distro? If not, are you capable of using a partition resizing tool?

Check out http://www.distrowatch.com for reviews, write ups and general info on the different distros out there. You will need to make a decision on what you want.
If you would prefer to not install a GNU/Linux distro, you could always test run with a LiveCD version. These are bootable CD's which contain a complete GNU/Linux distro, which work well.
You can run GNU/Linux this way without worrying about messing up your current system.
This way you can learn how to use the system, without any of the accidental mistakes which can happen when installing a system. Installing a system is not too difficult, but there can be some quirks which if you never had to configure Xfree86/Xorg, then you might be left scratching your head.

Running a LiveCD is also a good way to determine if most of the hardware in your box is supported, and is also much easier and faster to test run several different distros, until you find one which your comfortable with.

There are many different levels of GNU/Linux distro's out there as well. Some which cater to the beginner user to the most advanced users. Which is easier to get up and running by using 'wizards' to being left only to configure your system with text editing the config files with vi.

I've noticed that alot of the 'Windows' like distros that are quicker and 'easier' to setup, also seem to remove alot of the power of Linux from the user. This is ofourse a trade off and serves a purpose for beginners.

If you are fairly comfortable using DOS, then you may want to consider a distro with less wizard menus.

For me, since I am a fan of OpenBSD, FreeBSD, I wanted a Linux distro which was closer to *BSD than not. So I use Slackware Linux. *Bsd's and Slackware lack any wizards and requires alot of hand editing config files. But this makes setting up each system faster IMHO. Since I am able to set up all aspects of the system from a console or across a ssh session.
This is something that is difficult if not impossible to do on a system which relies heavily on GUI menuing systems.

Wizard menus are great for new users, or users which are still more comfortable with MS Windows.

Many new users looking for a more Windows like system, will test their hands with Mandrake-Linux.

But as stated, check out the site above, and if you have a fast internet connection, test out various LiveCD's distros.