TL;DR: Use Ubuntu. Least amount of pains for a Windows user to initially get set up and running, while giving you the ability to delve into it as deep as you want to as your comfort level increases.
Once you're familiar with one distro of Linux, you'll be more or less familiar with most of them. Some use different package managers, but otherwise are very similar, especially when running from shell.
The easiest distro to get up and running with (and one, if not the most popular) is Ubuntu. It's pretty much Linux for the Windows user. I suggest it as your introduction to *nix using this path should be relatively painless and you can start digging into the operating system more as your comfort level increases.
If you're able to hold your own building apps from source and using a shell rather than navigating through its file system explorers (trust me, it's faster once you really get it down) and can
grep/awk your way through a source tree, and you find yourself not being able to live without Cygwin on your Windows machine and would rather use Vim/Emacs to VisualStudio (at least when you're writing code), you're well on your way to being comfortable enough to use it in a work capacity :)
Not that I've seen many (if any) job posts about it, but do be aware that being familiar with a Linux distro helps, but doesn't mean that you're familiar enough with *BSD to use it effectively in a job capacity. Not only do the file systems differ, but the kernels are quite a bit different too.