Susy 0.9 Tutorial
See the official Susy site for 1.0 documentation.
For this tutorial I'm assuming you are already comfortable with CSS, Sass (I'll use the SCSS syntax) and Compass. Please get set up with each one of those before attempting to use Susy. Sass and Compass both have their own setup instructions and tutorials that you can use.
There is also reference documentation in the works.