I would recommend to give a LiveDVD/LiveCD a shot first before tinkering with your harddrive. From there on then, if you like the distribution you choose, search for tutorials on how to repartition your harddrive (Resizing NTFS filesystems is in Linux due to the closed nature of NTFS not always working) and then install it. Google is your best buddy for a guide (If you have trouble understanding it you are welcome to ask around, but not everybody nows an answer to every problem )
For a small overview of different distros take a shot at www.distrowatch.com. Don't take everything they say as a general truth. Linux is much about your own preference. I myself am running Gentoo and couldn't go back to another distro.
Ubuntu is great to get a glimpse of what Linux can offer in regards of userfriendliness. After that I might suggest switching to Arch and skipping Debian completely. Old packages and not always fast important updates. Arch is a rolling release distro as well, so you don't have to think like "In 2 years this version won't be supported anymore. Damn!"
A rolling release means that there is no "Arch v2.3.1 e.g." only Arch Linux. They don't make stuff like Ubuntu does with "Ubuntu 10.10" (can cause major bugintroduction every new release btw.).
Apart from that, have fun tinkering around and enjoying what FOSS has to offer

Cheers,

mikki-kun

Ohhh, I nearly forgot to mention it. I would keep my hands of Fedora for a while. They seem to have trouble with the most up to date Linux Kernel, thus tinkering around with it badly...