I hope this isn't too late, but as someone who exclusively uses Linux I wanted to help.
Linux has a lot of distributions you can get ("Linux" refers most specifically to the kernel itself). I think your main concern as you choose should be what will you be using it for? Keep in mind that on any distro, you will likely be able to install whatever programs you like--so, for instance, you install Ubuntu and it comes with LibreOffice and you don't like its word processor? No worries, you can always install something like AbiWord or maybe even run Word on Wine instead. The main practical differences between distros will be things like the type of window and display managers it uses, what package manager it uses, and how the install process works.
I agree with Stonedahl's suggestions for the most part, but if you're hoping to stick to more popular (and thus better-documented) distros, I would strongly suggest Mint, in whatever flavor, over the *ubuntus--it's a bit lighter, more user-friendly to someone who is used to Linux, and has better out-of-the-box support for things like playing MP3s and other proprietary file formats than Ubuntu last I checked. Mint and Ubuntu you can download and put on a disc or a flash drive to do a guided partition and install, so you don't really need to know what you're doing.
I'm an Arch user, and if you want Linux because you want to learn it or understand how OSes work, I cannot recommend Arch more, and I would always be willing to personally help you. The installation requires you to have Internet and gives you a blank command line, and then you do everything yourself. You will know everything about your system and be able to diagnose when something is wrong if you do this. Arch is incredibly well-documented via the Arch Wiki (they also have installation guides there--I used the Beginner's Guide!) and I recommend you turn to the Arch Wiki when you want to know stuff about Linux or Linux programs even if you end up installing another distro. If this sounds too overwhelming but you want the educational experience, you could also try doing it in its own small partition or on another computer sometime.
Also seconding Stonedahl's warning about backing things up. When I first installed Mint, I meant to dual-boot, but then my cat knocked my computer's plug out of the socket from the other side of the wall while I was partitioning. So, the unlikely can and will happen, so be safe!