If you haven't already got Inkscape installed, head on over to Inkscape.org
and download the latest version of Inkscape for your computer. Inkscape is great, not only is it free to use, it will work on Windows, Mac OS X, and most flavors of Linux.
Once you've downloaded the Inkscape application, follow the installation steps.
Once you've installed it, open up Inkscape so it's ready for the next step...