You can find installation instructions for both WordPress and ClassiPress on our theme documentation page
here. Your first step is to actually purchase hosting for your website (it will not work on a free WordPress.com account), and you may be interested to know that if you sign up via one of our preferred hosting partners you could receieve a 50% discount off your theme purchase. Please see more details
here.
Once your hosting is set up, WordPress is free and would need to be installed on your host account, after which time you would install the ClassiPress theme. Thanks.