You need to install ruby >1.9.3 and clone the repository. Open a terminal and cd into the repository directory. (if you have osx or linux you should already have that, on windows I recommend this http://rubyinstaller.org/)

Install all necessary ruby libraries:

bundle install

Once that's done, you are ready to contribute.

You can add/edit the files and compile it and start a small http server to look at it in your browser with rake serve.

this automaticatlly compiles all files when you make changes, so no need to restart the server

You can publish the site with rake publish.