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You can also go to www.distrowatch.com
for comparisons between distros.
Many distributions have Live CDs, which are working copies of the OS on CD that can be booted to, and that won't mess with whatever OS is on your hard drive. For instance, you can safely boot to the Ubuntu LiveCD on your Windows PC without killing your Windows OS.
My personal opinion:
- Ubuntu would be a good beginner level one. It asks relatively few questions during installation, and gets you running pretty fast.
- Then, jump into Fedora. More intermediate. Some different commands than what's in Ubuntu, so you'll learn.
- If you dare, check out Slackware. A bit more raw in its design, and forces you to learn much more.