C is a great language! It is fast, powerful and small enough that you can pick up most of the important concepts quickly. It's easy to get started, all you need is a plain-text editor and a C compiler!

GNU/Linux Setup

If you are using GNOME as your Desktop Environment, I recommend Gedit as your text editor. If you are using a Debian-based distribution, you can get it with:

sudo apt-get install gedit gcc

If you are using MATE, I recommend Pluma:

sudo apt-get install pluma gcc

That's all there is to it!

For other text editors a rather good list can be found here.

OSX Setup

The Developer Tools might have come with your Mac. If it came with CDs you can just install Xcode from the "Optional Installs" folder of your OSX installation media. If it didn't then I think you can download it for free from the App Store. You should now have GCC installed on your system (unless they've switched to Clang by now).

Windows Setup

I have used Bloodshed Dev-C++ in the past, which is pretty good. It includes MinGW as the compiler (a port of GCC, the GNU Compiler Collection). If you don't want an Integrated Development Environment (IDE) and just want the MinGW compiler, you can get it here.